Adding functionality for view to be a path to json file.

Up to you whether or not you want to merge this in. For a project I'm working on, I require taking a view context in from a file. Instead of requiring the fs module and doing it within the gulpfile, I decided to take your plugin and add the functionality. 

I also switched invocation to use es.through because it is not possible to emit error events from es.map. I added 2 tests: 1 using a correct json file and 1 using a malformed json file. The tests confirm that error events are thrown when reading malformed json from the file.