Rationalize cmd line arg parsing

[rneatherway]

We should split out the verb processing from the option processing to make things a bit better. We can also then generate the docs automatically from the usage. Paket does both of these things neatly: https://github.com/fsprojects/Paket/blob/master/src/Paket/Commands.fs

An alternative command line arg processor is https://github.com/gsscoder/commandline/blob/master/demo/fsharp-demo.fsx