Allow alternate variable names in import/export transforms

Resolves #143 and updated version and Readme accordingly.

I also added an "unknown" transform which will output an error message given any `///ts:???` where `???` is not a valid transform. There was a comment in CHANGELOG about adding such a feature and I figured I was already in there so I might as well do it.
:+1: however this is what is causing the test to fail (`_file` expected https://travis-ci.org/grunt-ts/grunt-ts/builds/34820718#L283). Just update the values in the `expected` folder. 

I had to do this `export/import` hack for an older version of the language service that crashed on `export/import` being on the same line.