Allowing multi language support for spell check

**Is your feature request related to a problem? Please describe.**
I find quite limitating the idea that the spellCheck (coming from [CodeMirror SpellChecker](https://github.com/sparksuite/codemirror-spell-checker) ) should only be on the American English dictionary.

**Describe the solution you'd like**
How hard would it be to fork, take into account the changes from the actually open (*and sadly stale*) PR https://github.com/sparksuite/codemirror-spell-checker/pull/39 adding multi-language support and depend on this library rather than the official spellChecker (which appears as stale as SimpleMDE) ?

An addition would be to, perhaps, make a map between locales and the dictionaries from [titoBozout's repository](https://github.com/titoBouzout/Dictionaries) so that we could simplify the spellChecker attribute to:
- false to disable totally
- `locale` (for managed dictionaries)
- `{aff: string, dic: string}` (for custom dictionary as is implemented in the PR linked above)

**EDIT**

I went on and forked SpellChecker to start what I had in mind ! :smiley: : https://github.com/TristanPouliquen/codemirror-spell-checker