Convert formatting.

[setpill]

Sourced from: https://modern.ircdocs.horse/formatting.html.

Format	IRC	Mattermost	Notes
Bold	0x02	**/__
Italics	0x1D	*/_
Underline	0x1F	Not supported
Strikethrough	0x1E	~~	IRC support only in Textual
Monospace	0x11	`	IRC support only in IRCCloud
Color	0x03	Not supported	Already stripped
Hex Color	0x04	Not supported	Coulnd't test; maybe already stripped?
Reverse	0x16	Not supported
Reset	0x0F	Non-trivial

Also something to take into account is that IRC format characters aren't paired; they are toggles. Only one is required to format an entire message (or starting from halfway in). In order to properly deal with this in mattermost, any message containing an odd number of the same format character (combination) should be terminated with that character in order to "close the tag".

The need for this makes the Reset character less work, since it is essentially the same functionality at an arbitrary point in the message.

[hloeung]

@setpill, or anyone else, I have something which seems to work for me. Would you be willing to help test this out? #546