tesseract: Allow to specify a subset of languages.

Especially useful for our OCR based VM tests, where we only need the
english language. By default the argument is null so all languages are
included. If a list of language name is passed only those languages are
enabled, for example:

tesseract.override { enableLanguages = [ "eng" "spa" ]; };

To only enable support for English and Spanish languages.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>