WIP - Rop experiment

#464 

Submitting to get some feedback. 
I've applied (at least tried to) Railway Oriented Programming to two commands: Convert from nuget and Simplify. The results / errors of a command are handled by call-site in PublicApi.
Pros
 - I find the code a bit easier to reason about 
 - No modification in file system is performed until the command returns success result

Cons
 - code overhead for the infrastructure
 - in order to fully benefit from the technique, probably all commands ought to be transformed to use it
When I was playing about computation expressions last year I made a very simple implementation of this: https://github.com/TWith2Sugars/AttemptIt Not sure how much help it would be.