Added support for specifying cursor

I wanted to be able to specify the cursor styling to use when using `Quintus.input.mouseControls()`. Just added a bit of code to check to see if the user has passed in a `cursor` parameter to `mouseControls()`. If it's not there, it functions as before, and sets the cursor to `none`. If the user just wants a plain old cursor, they can call it with `mouseControls({cursor: 'on'})`. If they want to specify a particular cursor they can likewise pass it in. (Something like: `mouseControls({cursor: 'move'})`.)

Quintus is a lot of fun, by the way! Just starting learning it.