Parameterize output module format

[mstade]

We currently assume AMD as the output module format, something which may or may not change down the line, but is crucial to make things run in a production environment. That said, in development, it may be useful to output different formats. For instance, running tests in a non-browser environment may well be easier with CJS rather than AMD. Additionally, in a development environment, globals may even offer development server capabilities such as reloading modules on the fly, much like figwheel does for ClojureScript.

So the proposal is as such:

• Add a --modules option with the following possible values: amd, cjs, umd, global, system
• Ignore the --modules option when combined with --production, in which case amd will be used

[mstade]

Come to think of it, maybe --modules should not be ignored in conjunction with --production, just that it should always default to amd... This would have the interesting effect that ez-build could build itself.

[mstade]

Closing because #23.