Allow for specific `appending` and `prepending` of files

I had the intentions of adding the ability to knowingly append and prepend a stream (honoring the order of the individual stream definitions), and that's what I did. But what I didn't expect is when I first pulled down the repo, `npm test` failed. It seems that the order when the streams are combined is not preserved, and it's not predictable how they might output. It's possible a dependency (i.e. `vinyl`) changed how it behaves to remove any sort of predictability (see #3)?

Anyway, I tweaked that test and added the ability to append and prepend files and maintain order within themselves (and added tests for those and updated the README).

Oh, and all this business about `gulp.src` doing this instead and this plugin isn't needed anymore (see #1)... seems to be hogwash? It doesn't seem to work at all. Maybe it's a bug or we're all missing something?