add ability to pass args thru browserify transform.  closes #5

This is basically @Nikku's PR #4 with whitespace changes removed, and updates to Browserify 7.0.0 for testing.

I made a small modification to his PR [here](https://github.com/boneskull/exposify/compare/thlorenz:master...boneskull:issue/4?expand=1#diff-168726dbe96b3ce427e7fedce31bb0bcR19) because somewhere between 4.x and 7.x the options object *always* contains a `_flags` variable.  So instead of using `EXPOSIFY_CONFIG`, `exposify.config` or `transform()` parameters, we always use the `transform()` parameters and "fill in the blanks" if our config is missing.  This *should* not break compatibility with 4.x/5.x/6.x, but of course I haven't tested against all versions.