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 had placed that variable there just to make it as easy as possible to go in and edit the path so it works wherever you pasted the code. But yeah, it's unnecessary; and especially if the code gets minified, users should be encouraged to use the option instead of manually altering the snippet, I suppose.