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
Merge branch 'transform-flags-opts' of https://github.com/hughsk/node-browserify
_flags
document transform opts, opts._flags
5.13.0