Refactor ISUPPORT into a component.

Basic things verified working without ISUPPORT enabled.  The only module
that requires it, afaict, is track.  More testing may be needed and is
welcome.
Add IsupportMixin to IRCClientCommonBase.
Update TODO, this is done.
Merge pull request #4 from Elizacat/isupport

Refactor ISUPPORT into a component.