Suggestion: use case-insensitive match for value of option `language`

[aviflax]

This just happened:

$ unibeautify --inplace --language markdown my-doc.md 
Language 'markdown' was not found. Did you mean:
  - Markdown

The “did you mean” is nifty, but a case-insensitive matching would have been even better here — it would have “just worked.”

(I’m also puzzled why the tool doesn’t just derive the language from the filename extension, but I guess that’d be a different issue.)

Thanks for the compelling tool!