Added ability to skip generating of existing files

[psynomorph]

Added the ability to skip recreating and overwriting already existing files by passing a special argument to the tool.

[psynomorph]

I'm using Mapster only for generating static mappers and extension methods. Sometimes I need to edit generated mappers to add more complex mapping logic in concrete mapper. Unfortunately Mapster tool rewrites all generated files on each run. In order to generate new mappers, I have to copy the old ones to another location so they don't get overwritten.
I thought it would be nice to have ability to generate mappers and extensions only for new models and interfaces.

[andrerav]

Thank you very much @psynomorph! I have updated the Wiki here to reflect this new functionality: https://github.com/MapsterMapper/Mapster/wiki/Mapster.Tool

[andrerav]

This is now available in the latest pre-release package.