Feature - #296 Setting configuration variables in .taskrc

- Now supports 'task config name value', 'task config name ""', and
  'task config name' to directly modify the .taskrc file.
- Updated man page.
- Added unit tests.
- Modified existing config command to also display configuration
  variables that have no values.

ChangeLog

@@ -26,6 +26,9 @@
    information.
  + The 'config' command now complains about use of deprecated color names in
    your .taskrc file.
+ + Added feature #296, that allows the 'config' command to modify your .taskrc
+   settings directly, with the command 'task config <name> <value>', or
+   'task config <name>' to remove the setting.
  + Task now supports nested .taskrc files using the "include /path" directive.
  + The 'entry', 'start' and 'end' columns now have equivalents that include the
    time, and are called 'entry_time', 'start_time', and 'end_time', for use in