[commands] Special handling for when the converter is bool.
[commands] Special handling for when the converter is bool.

Rapptz/discord.py,Harmon758/discord.py,variablehair/Eggplantato,mgardne8/discord.py,khazhyk/discord.py,imayhaveborkedit/discord.py,Theoretical/UselessBot,Harmon758/discord.py,jhgg/discord.py,rapptz/discord.py,lgkern/PriestPy