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 https://github.com/golangci/golangci-lint/issues/1809 )

[dmitshur]

FWIW maligned was refactored to implement an analysis.Analyzer in https://github.com/mdempsky/maligned/commit/c37866e2d38b8e33600b215e4095af870becd4d8 so there's precedent, but I'd wait to hear from the package author.