refactoring to split cli and library

[ryankurte]

hey thanks for making a neat thing! i wanted to try calling this as a library, so, had to do a bit of refactoring...

• first-pass moving things around to split the command line tool and library
• moves to enums for switchable functionality
• updates to a newer version of clap with derive to simplify all the argument parsing

i'm sure there are further improvements, at the moment all the internal functions are public and certainly not all in the best places, but this seemed like a good point to stop and open a PR and see if you're interested in the changes? ^_^

[ryankurte]

ping @brion