Exposing browserify requires

[jarnoux]

Hello @caleb, your plugin has been very useful and we have needed the require - expose API of browserify.

This small PR implements the following:

• exposes the browserify.require functionality
• makes entryPoints optional if the require option is specified
• adopts the option-passing API of the well-used grunt-browserify plugin
• maintains backward-compatibility with the previous { tr: ..., options: ...} transform API
• updates documentation accordingly

Thank you! Jacques

[caleb]

Cool!