Include unconvert as a library

[zaynetro]

Is there an interest on splitting unconvert into library and cmd? This will allow other projects like golangci-lint to use this project directly instead of depending on an old fork

I am ready to implement the changes myself. (Original discussion golangci/golangci-lint#1809 )

[dmitshur]

FWIW maligned was refactored to implement an analysis.Analyzer in mdempsky/maligned@c37866e so there's precedent, but I'd wait to hear from the package author.