allow -x to pass on the expose option

Similar to how you can use -r ./module-file.js:module-name to set the
exposed require name for a module, we should be able to set the exposed
module name for external modules.

This allows the user to require vendor libraries into a bundle using
short names, and to allow access to those libraries using the short
name.

An example using jquery installed with bower might look like:

browserify -r ./bower_components/jquery/jquery.js:jquery -o vendor.js
browserify -x ./bower_components/jquery/jquery.js:jquery app.js