Add de-duping functionality about classnames

[dcousens]

From my assessment, #21 is missing some edge cases because it doesn't split strings, so the following test would fail ...

That is what I found as well.
I think #21 is the best candidate, granted the tests could also be simplified.

As mentioned, we should pull the de-dupe implementation out to a separate file (dedupe.js) after merge.

[JedWatson]

PRs merged, ready to clean up and release.

[JedWatson]

Outstanding tasks to clean up dedupe:

• wrap with IIFE so that utility variables and _parse function don't bleed into the global scope when the script is simply included in a page
• move strict mode inside the IIFE so it doesn't affect other scripts when concatenated, ref https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode
• use Array.isArray as per change in 2.0.0
• should export classNames when used globally, it's a drop-in replacement for as per index.js

I'm on these now.

[JedWatson]

Just released 2.1.0 with this included.