Add support for prefix as a function

[thecrypticace]

Closes #159

Due to the implementation of the JIT we must support utility class prefixes in addition to the selectors themselves.

For example for .tw-ml-1 we'd pass .ml and .ml-1 to the prefix function. This is a byproduct of how we handle matching of classes before we generate the rules.

Because of this we'll want to encourage people to use a startsWith check instead of equality or includes:

prefix(selector) {
  if (['.align-bottom', '.ml'].some(prefix => selector.startsWith(prefix))) {
    return 'tw-';
  }

  return '';
},

You can also do this using an anchored regular expression which can be shorter:

prefix(selector) {
  if (/^\.(align-bottom|ml)/.test(selector)) {
    return 'tw-';
  }

  return '';
},

[adamwathan]

Thanks dude! 👊