Allow user specified formatting and colour for github webhooks

[jesopo]

[jwheare]

You'll probably need to:

• Switch to named params in format strings to let people omit/move stuff around.
• Move all utils.irc.color choices to named prefs
• Allow colouring more things

Appendix: Strings I'd personally want to potentially change, at first glance (click to expand):

_gh_issue:

https://github.com/jesopo/bitbot/blob/f431fdfd16d6959d30df67df5594275cc77bba55/modules/github/init.py#L105-L107

source:

https://github.com/jesopo/bitbot/blob/f431fdfd16d6959d30df67df5594275cc77bba55/modules/github/init.py#L366

commit_comment:

https://github.com/jesopo/bitbot/blob/f431fdfd16d6959d30df67df5594275cc77bba55/modules/github/init.py#L469

push:

https://github.com/jesopo/bitbot/blob/f431fdfd16d6959d30df67df5594275cc77bba55/modules/github/init.py#L441-L443 https://github.com/jesopo/bitbot/blob/f431fdfd16d6959d30df67df5594275cc77bba55/modules/github/init.py#L457-L459

pull_request:

https://github.com/jesopo/bitbot/blob/f431fdfd16d6959d30df67df5594275cc77bba55/modules/github/init.py#L494-L495

pull_request_review:

https://github.com/jesopo/bitbot/blob/f431fdfd16d6959d30df67df5594275cc77bba55/modules/github/init.py#L508-L509

pull_request_review_comment:

https://github.com/jesopo/bitbot/blob/f431fdfd16d6959d30df67df5594275cc77bba55/modules/github/init.py#L517-L518

issues:

https://github.com/jesopo/bitbot/blob/f431fdfd16d6959d30df67df5594275cc77bba55/modules/github/init.py#L528-L529

issue_comment:

https://github.com/jesopo/bitbot/blob/f431fdfd16d6959d30df67df5594275cc77bba55/modules/github/init.py#L542-L544

[jesopo]

I'd also like to add custom formatting for things like RSS - might be worth me putting some thought in to a more generalised templating system that can be used for both?