Mention roles & channels

[ynerant]

This pull request lets optionally to mention roles and channels through IRC. This also adds an option to mention @everyone and @here.

Issues that can be discussed:

• If the @ROLE is not at the beginning, at the end neither separated by spaces, the role should not be mentioned.
• Roles and channels are queried at each message. We could store locally a copy of all roles/channels.
• The structure of channels of Discord is used for mentions. But we can imagine that if multiple channels are bridged on a same guild, names are updated. For example, if I bridge #irc-a to #discord-a and #irc-b to #discord-b, mentioning #irc-a on IRC should display #discord-a on Discord and reciprocally. This is not obvious, and I don't think that it is really needed.

[qaisjp]

I think it would be better to always parse @everyone/@here/roles, but use https://discord.com/developers/docs/resources/channel#allowed-mentions-object to prevent the mentions from actually notifying people.