Mix deps.add functionality

[OvermindDL1]

@chvanikoff

Ok, that appeared to be more tricky than I thought - simple “read, parse/update and write ast” approach appeared to be not enough since all the formatting/comments/etc. is getting lost due to not being part of ast.

Yeah, the Elixir AST is heavily processed, it is not reversible like it is for other languages like C++ or so. However it is significantly more simple and easy to work with, just not reversible.