Revamp build system

[gkatsev]

Currently, the build system and output assumes adding vtt.js as a global but it's more useful to be able to import or require it using a bundler like browserify or webpack.
In the Video.js fork, I've made a lot of changes for this, also, to allow unshimming the global namespaces. We should probably work on making this module work better as modern javascript package.

[gkatsev]

You can see changes I've made here, starting with the browser-index.js file.