Add support for 'auto' language type which is available from LanguageTool API

[minoguep]

I noticed when looking at the the LanguageTool API documentation that this library doesn't support using the auto language code in conjunction with preferredVariants (see here).

Two changes were required here to support this:

1. Since the auto language code isn't returned by the GET /languages route the LanguageTag._normalize validation fails. To get around this I just add the 'auto' tag in the _get_languages function (and updated the test_langtool_languages test accordingly).
2. If we are using auto we can supply preferredVariants (e.g. en-US or de-DE). I just went ahead and added this to the LanguageTool object which can be set if required like:

tool = LanguageTool('auto')
tool.preferred_variants = ['en-US', 'de-DE']

Happy to make any styling changes or add any tests if required.

Cheers!

[jxmorris12]

This is a great addition, thanks @minoguep!