Rename `files` to `output-dir` (or something) in the config.yaml

[MageJohn]

I was surprised to notice the paths supplied to each member of the files map in an application config was actually a path to the directory that base16-universal-manager outputs the built files. I was expecting it to be the name of the actual file to output. The name seems misleading, so I suggest renaming it.

I'd prefer that it did work as I expected, too. That way I can give the file a descriptive name, rather than the usual default.ext.