Add support for initializing tags

This adds backwards compatibility for the old tags.
Continued work on the documentation
Added `destroy` method

Now Select2 supports destroying itself, and all elements that it
creates.
Started work on the classic theme

Some people like the old theme, so we're going to also include that
one as well as the easier-to-extend flat theme.
Change how the results are highlighted

Previously, a result was no longer highlighted when the mouse was
no longer over it. While this was useful for resetting the
highlight automatically, we started doing this when a new result
was highlighted to make sure that we never had multiple options
highlighted.

Now, a result will always be highlighted until the highlight is
removed when the next result is highlighted. While this shouldn't
affect many users, it is closer to how native select boxes work.
This also allows users using keyboard navigation to jump between
mouse and keyboard navigation without the highlight reseting to
the top of the results.
Better accessibility for multiple select boxes