Ignore charset AST

[vectart]

If CSS begins with charset, bless fails.

@charset "UTF-8";

[paulyoung]

Thanks @vectart!

@BlessCSS/contributors - it would be great to have everything in that switch statement where we return 0 represented in the tests (if not already) to give some confidence with those.

[mtscout6]

@vectart Can you add some tests that cover this so we don't regress on it in the future? This is per our Contributing Guide.

[paulyoung]

Relates to #86. It seems like we can't simply ignore @charset.