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!
I guess it's an opinionated assumption that people really do want to (or *should* want to) wait until DOMContentLoaded --- in which case I thought the advantage of using the listener is that it makes the snippet more portable, not dependent on placement --- somebody could paste it in the `<head>` and that would be fine.