Pass "flags" option to each transform

Essentially, `flags` is set to the value of the bundler's `_options` property.

This should enable more intelligent transform behaviours, for example toggling source maps when `--debug` is supplied through the CLI or module API.

* Closes substack/node-browserify#844
* Fixes hughsk/uglifyify#16
* Fixes hughsk/uglifyify#18

Thanks!