Pass `debug` flag through to transform options.

When using the debug option from either the command-line or module API, each transform stream's options will be modified to include `{ debug: true }`.

Likewise, false will be used if browserify is not running in debug mode. If a transform has specified its own debug option that value will be used instead.

I can see this is useful in a number of cases, but for me specifically it's to enable sourcemaps on glslify shaders when `--debug` is enabled.

Thanks!