Explain EXCLUDED_LANGS casing or make it case-insensitive #64

The current implementation expects the languages to be in the same style as displayed on GitHub e.g.

• HTML, not html
• Ruby, not ruby

My first instict when I set up EXCLUDED_LANGS was to make something like html,tex. Not until reading the source code I saw that the code expects the exclude list to be using the same casing as GitHub has. Thus to exclude HTML and Tex files, we must write HTML,TeX.

I propose one of these:

• We make the lookup case-insensitive i.e. normalize both each repo's languages and the user's set EXCLUDED_LANGS to lower-case.

  github-stats/github_stats.py

  Lines 350 to 351 in 9bcdc13

  	languages = await self.languages
  	if name in self._exclude_langs:

• If not, we should explain in the README that the EXCLUDED_LANGS should have languages as displayed in the generated image / as shown on the GitHub repo page