[punish] v2.0.0 modlog integration + numerous enhancements

Resolves #23 and closes #48

Changelog:

Commands were moved into groups. To use the original command names,
create global aliases for them.

The [p]punish list command has been updated to use a more compact table
layout, provided tabulate v0.8.0+ is installed.

Modlog integration is enabled by default for punishment durations longer
than 30 minutes. To change or disable this, use the [p]punishset case_min
command. The [p]punish reason command is also added to update reasons.

It is now possible to designate a special channel which punished users
are allowed to speak in. Instead of configuring this channel to deny
the punished role from speaking there, they are automatically granted
access to read and speak. To unset this, run [p]punishset clear-channel

Channel overrides can also now be customized and applied en masse. To
read the overrides from a channel, run [p]punishset overrides <channel>.
The channel can be either text or voice; the overrides for each type
are seperate. Server mute is not automatically applied if the voice
override does not deny speak or connect permissions. Once set, overrides
can be deployed to all channels with [p]punishset setup. To restore the
default overrides, run [p]punishset reset-override [channel_type], where
channel_type is voice, text, or both (the default).