Implement emitting utf8 charset when seen in output

https://github.com/sass/libsass/issues/629: "when we run into utf-8 characters, we throw in the charset"

There are some unknowns and open things here:
- Does ruby sass support other encodings than utf-8
- We don't, so anything else is out of perspective here!
- [x] ~~IMO this should be configurable via a context option~~ -> optional
- [x] ~~Code needs some re-factoring (use less copy paste)~~ -> todo #734
- [x] Needs to update spec/basic/48_case_conversion -> https://github.com/sass/sass-spec/pull/178