Add option to override existing release assets

[piranna]

By default, if a package is already published it get untouched. Add an option to allow to override them with newly built ones.

[vweevers]

@piranna Could you please clarify? By "package", do you mean GitHub release?

[piranna]

Don't remember the details, but yes, I thing it was about override the published release in GitHub repo so npm version gets untouched but a newer prebuild is fetch, for example for development purposses.