Add make ONLY flag: 'make ONLY=Arrow test/features' to run Arrow tests

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/794

Closes #794.
All expressions done
wip
Refactor pretty printing to handle the white space at the AST level

We previously determined when to add the whitespace at the token level. That lead to a bunch of complicate switch statements. With this we handle the formatting at the AST level which simplifies the code significantly and it also allows us nicer result.

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/795

Closes #795.
wip