build: don't regenerate/copy files that have not changed

[KajKandler]

When building my static site a lot of files (assets) are copied into the dist folder, that have not changes what so ever. This makes it hard to deploy a site efficiently (for example via rsync).

It would be great to avoid such updates and just overwrite files that have changed.

[johnfraney]

It would be!

[johnfraney]

@KajKandler I just merged a PR that will help quite a bit with this. There are still some improvements that could be made here, but now Blurry won't copy files into the build/dist folder if those files are already there

#98