Commands

- Implemented CmdHelp object that replaces the report.cpp longUsage
  function, and builds the output dynamically from other Command
  objects.  This is also why the help text right now is very short,
  as only a few commands are migrated.
- Obsoleted longUsage function.
- Updated task.1 man page with 'execute' command details.
- Modified command.lua sample to include command usage.
- Removed "help" from old Context::dispatch, which means "help" is
  the first migrated command.
- Added usage and description to all Cmd* objects.
- Implemented Command::usage and Command::description as base class
  methods that simply return data that is specified by the derived
  classes.

doc/man/task.1.in

@@ -209,6 +209,12 @@ to achieve this:
 This example first gets the IDs for the project:Home filter, then sets
 the priority to H for each of those tasks.
 
+.TP
+.B execute <external command>
+Executes the specified command.  Not useful by itself, but when used in
+conjunction with aliases and extensions can provide seamless extension
+integration.
+
 .TP
 .B version
 Shows the taskwarrior version number
@@ -217,7 +223,7 @@ Shows the taskwarrior version number
 .B help
 Shows the long usage text.
 
-.TP 
+.TP
 .B show [all | substring]"
 Shows all the current settings in the taskwarrior configuration file. If a
 substring is specified just the settings containing that substring will be