Add MODE support

[ldilley]

Handle MODE command and all its various characters and flags for umodes and channel modes.

MODE support is mostly in. The only items remaining are:

1.) Check for access level (admin, founder, chanop) when a user attempts to change channel properties and allow/deny appropriately.
2.) Handle chanflags/prefixes for users.
3.) Handle ban, chanop, and voice adds/removes.

The on_mode() method may also require a rewrite as it is a mess/is long.

[ldilley]

This issue is now priority.