Add checktextdomain task

This is a first pass.

This is a fork from  https://github.com/stephenharris/grunt-checktextdomain and the majority of the commit is from code Stephen Harris wrote.

This commit:

* Changes option names to be camel case instead of underscores, as per common practice. Fixes https://github.com/stephenharris/grunt-checktextdomain/issues/2
* Allows `createReportFile` to accept a file path string instead of just being a boolean. This is used within the unit tests to keep the generated report files within the `tmp/` directory. This directory is already cleaned before testing, so it avoids having to tidy up each file during the testing as it was in the original code.
* Passes all existing unit tests. No functional testing done yet.
* Tweaks and merges existing documentation into `README.md`.

More work will be needed on making the options more compatible between this and the existing tasks, and that functionality doesn't overlap, but this should be a good start.