Add a copy option to sift task

[kenbier]

Sift currently supports an option for moving files in the fileset, but it'd be nice to copy files to another place within the fileset. The original source of the transformation would be left unchanged by this option.

Something like:

(boot sift :copy #{"^public/" "foo/"}

[burn2delete]

There is also https://github.com/cpmcdaniel/boot-copy