Output directory now used by target_file_name

[dsteeley]

Fixup bug from clap migration where the target file name was determined using the target argument and the docs indicate that it should be determined using the output argument.

[dsteeley]

Fixes #90

[juliusl]

There's a bit of non-trivial logic going on here, could you extract this into a fn and add some unit tests?

[dsteeley]

There's a bit of non-trivial logic going on here, could you extract this into a fn and add some unit tests?

Done, added a test for each of the cases in that logic.

[cat-in-136]

Thanks a lot.