A start on loader script.

This is a start on the loader script discussed in https://github.com/shakyShane/gulp-svg-sprites/issues/52.

I created a dust template for `loader.js` and a couple of additional options to customize the filename and the loading path.

I also added to the current tests and changed a few sentences here and there in documentation.

I'm not sure this is ready to be finalized yet but more of a draft. I was looking for your feedback before putting more into it.

Some of the things not quite finished:

- Should uglify/minify the generated `loader.js` so that it's ready to copy-paste directly into the document head, or some similar place.
- Should I add more documentation about this and how to use it, or do you think this is enough? 

So do you think this is on the right track? Thanks!
Is there a list of browsers that we know to support this kind of SVG inclusion (defs and symbols only)? I didn't see one documented. That would be a start ... In [the first CSS-Tricks article about this technique] @chriscoyier has note about browser support --- and I think all of those browsers should also support XMLHttpRequest.