Add guessing game example to showcase wait_for_message.
Add guessing game example to showcase wait_for_message.

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