[commands] Make Bot.cogs and Bot.extensions read-only mappings.
[commands] Make Bot.cogs and Bot.extensions read-only mappings.

This also has the side effect of effectively documenting them for
public use.

Harmon758/discord.py,rapptz/discord.py,Rapptz/discord.py,imayhaveborkedit/discord.py,khazhyk/discord.py,Harmon758/discord.py