Support usage as a library

[frenata]

See #2. I'm hopeful this will actually be a relatively minimal change, mostly involving reviewing what is and isn't exported.

[frenata]

box = compilebox.New("data/compilers.json")

http.HandleFunc("/languages/", getLangs)
http.HandleFunc("/eval/", evalCode)

At a minimum, these are the functions that need to be "exposed" for use as a library.

New exists as well, New.

evalCode exists as EvalWithStdins, though this begs the question of whether there should be a plain Eval We should think hard about the actual API for that function too, as well as how errors should be handled.
There might be some fun higher-ordered function stuff that can be done with Eval too. :)

getLangs pulls from an exported field LanguageMap and then does some manipulation, so that functionality should probably be extracted into a method on the box itself.
Since we have enabled/disabled fields in the language map, it would also be nice to expose an API to modify this on the fly.