Add spellchecking

[schlessera]

There should be a spellchecking test that verifies that documentation and docblocks don't contain any typos.

Language should be configurable per set of files/folders to allow for the translated pages in the https://github.com/wp-cli/wp-cli.github.com repository to be checked in the correct language.

Exceptions/custom dictionaries should be configurable both centrally in wp-cli/wp-cli-tests as well as on the target repository.