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
Maybe a custom operator would even be better - but only if everybody agrees on the same operator.