Add site spellchecking task

[Floppy]

This can be done with mdspell (see https://github.com/openpolitics/manifesto/blob/master/Rakefile), though it's far from ideal.

It should be an optional travis task and use a dictionary file, as the example above does.

Unfortunately the default aspell dictionaries differ hugely on different platforms; if there's a better cross-platform solution, I'd like to find it.

[DirtyF]

FWIW Middleman has https://github.com/minivan/middleman-spellcheck which is based on the spellchecker gem and aspell.

[Floppy]

Oh, useful @DirtyF, I'll take a look see if their approach works better.