Allow user specified formatting and colour for github webhooks

[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?