Dispatch a cog unload event [on_cog_remove]

[npc203]

Description of the changes

This PR adds an extra event akin to it's counter-part (on_cog_add). It dispatches when a cog is (successfully) unloaded with the cog object as it's only argument.

Thanks in advance!

[npc203]

Erm, just noticed, the other doc string line edit was done by black and not me