PEP 8 imports tidyup.

Also remove trailing whitespace.
Remove empty test method.

This is also syntactically invalid, and shows up as a failure.
Add testing via input and output YAML files.

This makes adding cli level tests as easy as definig the inputs
and outputs.
Correct behaviour when no arguments are provided.

My change in a9caaa487f754ce doesn't comply with the Proton spec,
which says that we should read from stdin in this case. Move the
help handling to a specific -h/--help option and read from stdin
if there are no arugents.
Add a test for the case of a nonexistent file being passed.

This falls under Proton's bad input response.