`harp compile` needs an option to not clear files

[geppy]

harp compile has the nice feature where it clears the output directory before filling it up. This is very convenient, except when you don't want it to do that for some reason or another. Would it be possible to add an option to not clear the output directory?

[kennethormandy]

Hey! Thanks for opening an issue. I agree, and I’m going to close this issue in favour of #253 If it’s possible to get the existing tests to pass with that option, I think we would be able to merge that pull request.