Add kern utility

[benface]

Thank you for the PR! Just a few things:

• Why is font-feature-settings: 'kern' required in addition to font-kerning? It looks like all the big browsers other than IE11 support font-kerning, so is it only for IE users? If so, personally I don't think it's worth it for a progressive enhancement feature like this.
• I think all utilities should be "undo-able" (especially for responsive purposes), so what do you think about having kerning-auto, kerning-normal, and kerning-none? Similar to the existing hyphens utilities generated by the plugin.
• On that note, any reason you went with kern instead of kerning? I feel like the latter is clearer/more widely used and would be a better name for the option and the utility prefix.
• Lastly, I apologize, but I forgot to mention that the latest work has been done in a next branch (not yet released, I was waiting for Tailwind 1.2), so if by any chance you can rebase your work against that I'd be grateful. If not I can merge it later. 👍

Thanks again!

[mapgrid]

• Yes, I think you're right on not needing font-feature-settings: 'kern'.
• kerning is better for sure.

[mapgrid]

Superseded by #15.