Add checktextdomain task

[GaryJones]

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.

[bradyvercher]

Nice work, but I'd prefer to see some of these improvements made in the original project if @stephenharris is still maintaining it.

[shivapoudel]

@bradyvercher the project of @stephenharris grunt-checktextdomain has no updates since v0.1.1 so be updated with this PR in this grunt-wp-i18n

[stephenharris]

@shivapoudel grunt-checktextdomain is still under active development insofar as that it requires bugfixes or there are in-scope features requested.

[shivapoudel]

@stephenharris I was unable to see any commits or other release to grunt-checktextdomain, sorry for that.

[bradyvercher]

Thanks for dropping by and letting us know your project is still active, @stephenharris! I'm going to close this out and let these updates be hashed out over there if @GaryJones wants to submit a PR.