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-config.1.adoc

@@ -0,0 +1,37 @@
+= timew-config(1)
+
+== NAME
+timew-config - get and set Timewarrior configuration
+
+== SYNOPSIS
+[verse]
+*timew config* [_<name>_ {_<value>_|*''*}]
+
+== DESCRIPTION
+Allows setting and removing configuration values, as an alternative to directly editing your ~/.timewarrior/timewarrior.cfg file.
+
+== EXAMPLES
+For example:
+
+    $ timew config verbose yes
+    $ timew config verbose ''
+    $ timew config verbose
+
+The first command sets 'verbose' to 'yes'.
+The second sets it to a blank value which overrides the default value.
+The third example deletes the 'verbose' setting.
+
+When modifying configuration in this way, interactive confirmation will be sought.
+To override this confirmation, use the ':yes' hint, which means you intend to answer 'yes' to the confirmation questions:
+
+    $ timew config verbose '' :yes
+
+If no arguments are provided, all configuration settings are shown:
+
+    $ timew config
+    verbose = yes
+    ...
+
+== SEE ALSO
+**timew-hints**(1),
+**timew-show**(1)