commands working on the selection

[ratmice]

It'd be nice to have "parse selection", and "generate parser from selection".
These would be nice for running the parser on markdown code blocks, and things like yacc files embedded in rust source code.

The main issue is for the former, which parser do we parse it with, because the selection doesn't have an extension.
My thoughts are:

1. Add a default key to nimbleparse.toml's parsers
2. Add a Set default parser command which works on yacc files, and presumably a set default parser to selection command.