Support for subfile, and reduction of errors thrown by grammar checkers

[sophiehuiberts]

I have tried to preserve sentence structure of sentences with math in them a bit. The current solution is not very good, but it reduces errors thrown by Grammarly a lot. I replaced math environments by the word "noun", and verb symbols (such as =, <, \in) by "verbs noun". Examples:

Then $P$ is non-degenerate. -> Then noun is non-degenerate. So $x \in P$. -> So noun verbs noun.

[pkubowicz]

Hello Sophie, thank you for your pull request.

Would you additionally add a test in a similar way I've done 66575a73? This is for two purposes:

1. to have a regression test
2. to have a specification-by-example helping to understand what effects your feature has

[sophiehuiberts]

Done.