Adds function for trac-admin to add comments to tickets.

The code is based on tracopt/ticket/commit_updater.py of trac.

Install the plugin into your trac project's plugins dir. To enable the plugin add this to the trac.ini file:

[components]
comment_add.* = enabled

Syntax: trac-admin comment add
Example: trac-admin /home/trac comment add 12345 'name email@example.com' 'Hello World!\nThis is a comment'