Add option --require to support node require() calls for modules=register

Pass relativeRequire as argument to module instantitor.
ModuleTransformer adds 'require' parameter to module instantitor.
Fixes #1486

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/1507

Closes #1507, closes #1486.
Fix FindVisitor to not start the visiting in the constructor

We used to call visitAny in the constructor, which lead to bad
API where we had to set things on this before calling super.
Add support for union type syntax