Files copied, but not in Metalsmith's database

[RobLoach]

While the files are copied through the file system, once in, they're not processed by Metalsmith's system.

The symptom is from using fs.copySync:
https://github.com/philgs/metalsmith-assets-improved/blob/release/index.js#L111

The other option is to allow injecting the files from the source directory, into the files database, so that plugins after can act on the installed files.