Add output_field_separator option
Add output_field_separator tests
Change structopt req to 0.3
Separate negative choices into a function
Prevent tail printing output_field_separator
Change OFS to Option<String> with a default value of " "
Reorder arguments to write_bytes to parallel print_choice
Print output_separator in main loop if applicable
Add `cargo test` to Makefile
Add write_separator function
