Add mattermost service command for handling channel headers

[kot0dama]

Considering IRC limitations (topic max length, IRC message max length, inability to set topics for private conversations), it could be interesting to have a mattermost service command for handling channel/DM headers.

For example:

/msg mattermost header get <channel/username>
/msg mattermost header set <channel/username> <new header contents>
/msg mattermost header update <channel/username> <sed-style or perl-style replacement s/blah/blip/>

I'm not sure of the limit for Mattermost headers, but the set command might be a bit tricky if we want to allow longer headers than what a single IRC message line allows. Hence the update command to allow using regex replacements :)

Thank you!