Permit both theme and custom classes

[mhuggins]

What's this PR do?

Permits both themes and custom classes. Prior to this pull request, it was not possible to have a theme take effect if you were also trying to pass in any custom classes, e.g.:

$('.token-autocomplete').tokenInput({
  theme: 'bootstrap',
  classes: {
    tokenList: 'token-input-list-bootstrap form-control'
  }
});

How should this be manually tested?

Try all four possibilities to ensure they work as expected.

1. No theme or classes option passed in.
2. Just a theme option passed in.
3. Just a classes option passed in.
4. Both a theme and classes option passed in.

Any background context you want to provide?

I'm trying to use a theme, but I'd also like to attach an unrelated bootstrap class to the rendered token input.

Screenshots

What gif best describes this PR or how it makes you feel?

[mhuggins]

cc @loopj @hugohenrique

[hugohenrique]

:+1: