Allow multiple settings objects to be used within a single page

I've added set-specific variable names for each of the settings objects. When you include multiple sets in your application.js file, this change allows you to differentiate between the settings objects. This is necessary when you want to have multiple MarkItUp editors on a single page, each using different sets.

(This is a backwards-compatible change: I've left the existing "mySettings" variables in place.)

The only downside I can see is that, if and when this gem needs to be upgraded with new set files, these changes will need to be re-applied. I consider this useful enough to be worth that overhead; hopefully you do too.