Stabilize CLI API

[LukasKalbertodt]

The API should make sense and be (more or less) stable at one point.

• [x] What about passing no files (all files in current dir?)?
• [ ] Some kind of javac proxy? What would this be used for?
• [x] What happens when no command is given (e.g. jswag Hi.java)?
• [ ] Semantics for watch command

[LukasKalbertodt]

What about passing no files (all files in current dir?)?

Yes. Passing no files is the same as passing ..

What happens when no command is given (e.g. jswag Hi.java)?

Currently it works as it does with jswag build