(feat) adds azure cdn support + tests

[cadriel]

This adds;

• The ability to define a custom domain. If set, this changes the host of the returned function getUrl. Mostly this is handy when using CDN's as they usually have a different domain.
• The ability to set cache control for Azure uploads. By default, these are not set in this adapter - so I've added the ability to do so. This is REALLY important for CDN's, but should probably be set in any case.
• The ability to set purging on and off for Azure CDN's. Unfortunately (unlike Akamai) Azure do not auto purge on overwrite or delete. This adds the ability to toggle purging on, and configure the required credentials to do so.

I've also added a .editorconfig for sanity's sake.

Finally - the tests in master were not working, nor relevant for the code. I have re-written a suite of unit tests.

[cadriel]

@wmertens @jossmac @JedWatson