[commands] Make sure that mentions are the entire string.
[commands] Make sure that mentions are the entire string.

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