Replace roff man pages with asciidoctor

This replaces the generation of man pages on project setup
by a on-demand generation via asciidoctor.
An exception are the man pages for the commands `day`, `month`, and `week`
which are simply redirects to the man page `timew-chart.1`. Those are now
static files in the Timewarrior repository.

A CMake find module to detect asciidoctor was added.
If asciidoctor is found, the targets `doc`, `man1`, and `man7` are created.
Those targets are also added to the default build target.

If asciidoctor is not available, the target `doc` is available, but it only
emits a message to install asciidoctor first.

Signed-off-by: Thomas Lauf <thomas.lauf@tngtech.com>

doc/man1/timew.1.adoc

@@ -0,0 +1,164 @@
+= timew(1)
+
+== NAME
+timew - a command line time tracker
+
+== SYNOPSIS
+[verse]
+*timew* [*--version*|*--help*]
+*timew* [_<command>_ [_<arg>_**...**]]
+
+== DESCRIPTION
+Timewarrior is a command line time tracker.
+It allows you to easily track your time and generate summary reports.
+
+This is a reference, not a tutorial.
+If you are looking for a tutorial, check the online documentation here:
+
+    https://timewarrior.net/docs/
+
+When run without arguments or options, the default command is run, which indicates whether there is any active tracking, and if so, shows a summary, then exits with a code 0.
+If there is no active time tracking, exit code is 1.
+
+== OPTIONS
+
+*--version*::
+Displays Timewarrior version information
+
+*--help*::
+Displays Timewarrior usage information
+
+== Timewarrior commands
+Timewarrior supports many commands.
+Alphabetically:
+
+*timew-annotate*(1)::
+    Add annotation to intervals
+
+*timew-cancel*(1)::
+    Cancel time tracking
+
+*timew-config*(1)::
+    Get and set Timewarrior configuration
+
+*timew-continue*(1)::
+    Resume tracking of existing interval
+
+*timew-day*(1)::
+    Display day chart
+
+*timew-delete*(1)::
+    Delete intervals
+
+*timew-diagnostics*(1)::
+    Show diagnostic information
+
+*timew-export*(1)::
+    Export tracked time in JSON
+
+*timew-extensions*(1)::
+    List available extensions
+
+*timew-get*(1)::
+    Display DOM values
+
+*timew-help*(1)::
+    Display help
+
+*timew-join*(1)::
+    Join intervals
+
+*timew-lengthen*(1)::
+    Lengthen intervals
+
+*timew-modify*(1)::
+    Change start or end time of an interval
+
+*timew-month*(1)::
+    Display month chart
+
+*timew-move*(1)::
+    Change interval start-time
+
+*timew-report*(1)::
+    Run an extension report
+
+*timew-resize*(1)::
+    Set interval duration
+
+*timew-shorten*(1)::
+    Shorten intervals
+
+*timew-show*(1)::
+    Display configuration
+
+*timew-split*(1)::
+    Split intervals
+
+*timew-start*(1)::
+    Start time tracking
+
+*timew-stop*(1)::
+    Stop time tracking
+
+*timew-summary*(1)::
+    Display a time-tracking summary
+
+*timew-tag*(1)::
+    Add tags to intervals
+
+*timew-tags*(1)::
+    Display a list of tags
+
+*timew-track*(1)::
+    Add intervals to the database
+
+*timew-undo*(1)::
+    Undo Timewarrior commands
+
+*timew-untag*(1)::
+    Remove tags from intervals
+
+*timew-week*(1)::
+    Display week chart
+
+== MORE EXAMPLES
+
+For examples please see the online documentation starting at:
+
+<https://timewarrior.net/docs/>
+
+Note that the online documentation can be more detailed and more current than this man page
+
+== FILES
+
+~/.timewarrior/timewarrior.cfg::
+    User configuration file.
+
+~/.timewarrior/data/YYYY-MM.data::
+    Time tracking data files.
+
+== pass:[CREDITS & COPYRIGHT]
+Copyright (C) 2015 - 2018 T. Lauf, P. Beckingham, F. Hernandez. +
+Timewarrior is distributed under the MIT license.
+See https://www.opensource.org/licenses/mit-license.php for more information.
+
+== FURTHER DOCUMENTATION
+For more information regarding Timewarrior, see the following:
+
+The official site at <https://timewarrior.net>
+
+The official code repository at <https://github.com/GothenburgBitFactory/timewarrior>
+
+You can contact the project by emailing <support@gothenburgbitfactory.org>
+
+== REPORTING BUGS
+Bugs in Timewarrior may be reported to the issue-tracker at <https://github.com/GothenburgBitFactory/timewarrior/issues>
+
+== SEE ALSO
+**timew-config**(7),
+**timew-dates**(7),
+**timew-dom**(7),
+**timew-durations**(7),
+**timew-hints**(7),
+**timew-range**(7)