Add machine readable mode

[ashpil]

While working on the gator study, it has come to my attention that gator could benefit from a machine-readable mode, i.e., one where another program can programmatically interact with the gator compiler.

Currently, if one wanted to programmatically compile gator code, one would have to create a temporary file, and then pass that file to gator. Also, if one wanted clean gator-only exceptions, one would have to manually trim the OCaml Failure ... message.

It would be nice if instead of creating a temporary file, one could just either: pass the gator code to compile as an argument, or, pass it to stdin. In this mode, the errors would be trimmed to gator-only messages and sent to stderr. For using ... queries, it would just look for files relative to the gator process's current directory.

I noticed parcel-plugin-lgl also mentions that such a feature would be nice.