Preserve order of files when using Stdout

[menelaos]

Currently

stylus -p file1.styl file2.styl

does not always print the resulting CSS in the order of the files
given as arguments.

The issue is probably with the asynchronous reading of the files in
the compileFile function.

This is not a problem when writing to a file since it doesn't really
matter whether file2.css is written before file1.css.

For using pipes however and for the sake of predictability, it would
be nice if this could be fixed.