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!
Do we need to wait for DOMContentLoaded. Can't we just tell people they need to put this snippet before the closing body tag?