[proposal] Use requirejs in development

[aerhard]

The current modular design of mei2vf has got one disadvantage: You can't inherit across files at an early stage, i.e. when the closures in the files are executed.
With the current design, when you want to inherit from a class on closure-level, you have to define the superclass in the same file, that's why there are multiple classes in https://github.com/TEI-Music-SIG/MEItoVexFlow/blob/master/src/PointerCollections.js, for example.
Requirejs would solve this in a much better way. Another advantage could be that you should be able to include xmldom client-side (see issue #17). I gave requirejs a try with mei2vf and it looks promising to me. Mei2vf's build process could even be designed in a way similar to jQuery's where the built files don't depend on requirejs. What do you think about using requirejs?

[aerhard]

Here's a demo with requirejs in which I also added the google closure compiler: https://github.com/aerhard/MEItoVexFlow/tree/require-js -- structure in /src is not final, but it should be enough to get an impression. Do you think it would be worth further exploring this path?

[aerhard]

(dist/meitovexflow.js and dist/meitovexflow.min.js are built anew; ignore that meiview's code is in the closure and then exported to the window object -- I think that shouldn't stay this way)