Add Role.mentionable attribute.
Add Role.mentionable attribute.

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