Consider spelling/grammar/etc. checking

[Infernio]

We'll probably want to use LanguageTool. There's a python wrapper that we should try.
But most important is thinking about how we can represent this AST mess as plaintext for LanguageTool to operate on, and then map that plaintext back to the AST nodes so that we can accurately highlight in the source code where errors came from.

Also:

$ time hunspell -l /tmp/test.txt
[...]
real    0m0.065s
user    0m0.062s
sys     0m0.004s
$ time languagetool --language en-US /tmp/test.txt
[...]
real    0m6.373s
user    0m20.568s
sys     0m0.590s

So we can certainly afford to do automatic spell checking, but grammar etc. checking should be reserved for a special command (zoia check, probably) and should download languagetool only then (considering it's like 200MB in size).
Edit: actually, I want zoia build to be as fast as possible - turn code into output (HTML/plaintext) ASAP. zoia check can afford to be slower.
Also think about how to use the system languagetool when possible.

Not a v0.1.0 thing, spellchecking can be added much later.