[commands] Allow customising the Context class in get_context.
[commands] Allow customising the Context class in get_context.

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