Add Command.cmdset_source

[volundmush]

Brief overview of PR changes/additions

This simply modifies CmdSet.add() to stamp each newly added command with cmd.cmdset_source = self

So, Commands now know EXACTLY which CmdSet they came from.

Motivation for adding to Evennia

This allows a Command to reference properties, methods, state, etc, on their CmdSet, which can be useful in a number of ways.

See, for instance, creating a kind of pseudo-menu which is aware of all of the commands available to the 'menu'.

Other info (issues closed, discussion etc)

This is such a tiny - but useful - change, it doesn't appear to cause any issues.

[Griatch]

I dislike the use of cmdsets for data storage. But I like the notion of storing the source-cmdset on the cmd for debug purposes. This may be useful in combination with expanding the examine command to show where each available cmd is actually coming from.