Added support for configurable loop

Now you can pass option to the constructor:

    loop: 'onlyNext|onlyPrev|both'

When `onlyNext` is set, slider will loop to the first slide when `next()` is invoked, otherwise it wont.
When `onlyPrev` is set, slider will loop to the last slide when `prev()` is invoked, otherwise it wont.
When 'both', that means both options below. 

Needed it in my project, so maybe someone else will benefit from this.