[commands] Add BotBase.get_cog_commands to get all a cog's commands.
[commands] Add BotBase.get_cog_commands to get all a cog's commands.

Self-explanatory. This should help create help commands for a cog
more easily.

khazhyk/discord.py,Rapptz/discord.py,Harmon758/discord.py,Harmon758/discord.py,rapptz/discord.py,mgardne8/discord.py,imayhaveborkedit/discord.py