Convert IRC formatting / emphasis to markdown for Mattermost

[hloeung]

This fixes https://github.com/42wim/matterircd/issues/258

Per https://github.com/42wim/matterircd/issues/258, this converts formatting/emphasis of IRC messages to markdown to Mattermost. Currently supported are bold, italics, bold+italics, monospace, and reset.

Testing with irssi (which doesn't seem to let me test italics and monospace):

> Testing BTestingB Testing BTestingO Testing (reset)

(where B = Ctrl+B and O = Ctrl+O)

Produces:

Testing Testing Testing Testing Testing (reset)

[hloeung]

Added stripping of reverse and hex colors, so this covers all the cases as per https://github.com/42wim/matterircd/issues/258

[42wim]

Not tested yet by me, but looks good :)