stored prefix and message_index to camel case

[TomOrth]

For discord bots, its better practice to store the prefix into a data storage such as a variable, file, or database, as then you can update the prefix the whole bot from one source. Also, its cleaner code to use camel case rather than underscores

[ErikBoesen]

I'd actually disagree with the latter point. Camel case is the standard for Java and JavaScript, however the reigning standard for Python is to use_underscored_variables. This is consistent with discord.py's standards, as well.