Rework inclusion of vtt.js

[gkatsev]

Description

We currently include vttjs by concatenating it to the release file. We also allow it to be included via a url in the novtt alternative build.
With the new build system coming from #3445 we probably want to include it another way that works with bundlers properly while still maintaining the ability to produce the novtt build.

[gkatsev]

So, I think what we want to have happen that we require vttjs directly but then in the dist builds, specifically, the novtt builds, it should be removed to use the asynchronous loading mechanism.

[brandonocasey]

perhaps by using the envify and unreachable-branch-transform browserify transforms

[gkatsev]

Fixed.