Fix DOCSTRING of js3-parse-switch.
Use the let-bound variable `char`.
Introduce `js3-label-indent-offset`, defaulted to zero.

For example, setting it to -1 will result in the following format:

    switch (state) {
     case 1:
      doA();
      break;
     case 2:
      doB();
      break;
     default:
      return;
    }
Merge pull request #90 from knu/label-indent-offset

Introduce `js3-label-indent-offset`.