IRC - Allow setting user modes

[half-duplex]

Summary

The bot should have the ability to set user modes on itself, notably +B (bot).

Background

Is your feature request related to a problem? Please describe:

Many networks request that non-human users set mode +B on themselves to indicate that they're a bot. This doesn't currently seem to be possible in TeleIRC.

Describe the solution you'd like:

TeleIRC should have a config parameter like IRC_BOT_MODES="+B" – possibly even have +B be the default – and should set these modes on itself after connecting.

Describe alternatives you've considered:

IRCv3 has a draft for bot mode, which should be implemented in the future when it's graduated from draft status and has wider support.

Details

inspircd3 botmode plugin Sopel, a python IRC bot framework, sets mode +B by default. docs

Outcome

TeleIRC users are able to easily or automatically abide by network operators' request to set mode +B on non-human clients.

[half-duplex]

What timing - the IRCv3 bot mode spec was ratified today. It'd be good to support this at some point, but user modes are probably easier and satisfy my needs at the moment. https://github.com/ircv3/ircv3-specifications/pull/495

[justwheel]

User modes make sense and would be a nice addition to TeleIRC.