New header, PEP8, some minor refactors, streamline arguments

 New header: Add hashbang and encoding lines. Include metadata and copywrite notice.

 Minor refactoring: Import specific functions from re and os, move infile code below si_score() function, renaming some variables in the function for PEP8 compliance.

 Streamline args: I switched to a single positional argument for sequence input, instead of requiring either the -f or -s flag. Included the logic to test whether a file is being passed in or a raw string, and handling as appropriate.