Support for loading multiple files

I added support for loading multiple files from different locations by passing a `files` key containing an array of objects. See **c0662dc** for an example.

I reused the previous code, wrapping it in a `load(file)` function. In order to remain BC while keeping code simple, if only an object is passed, it is inserted into the `files` array. Then, it loops through every file and waits for all promises to resolve before merging the returned data into a single object.

Again, **this change is BC.**

Questions? Comments?