sort input files like they were defined in joinTo

This is the first direction into stabilizing the input order of files.
Currently, files in joinTo are ordered alphabetically before being concatenated.

A second commit would fix [`sortByConfig`](https://github.com/brunch/brunch/blob/master/src/fs_utils/generate.coffee#L20) to not destroy the input order of the files given. This fix however probably needs to be made in [anysort](https://github.com/es128/anysort)

Closes #825