Use fs.copyFile() api in node 8.5+

[kanongil]

See https://nodejs.org/api/fs.html#fs_fs_copyfile_src_dest_flags_callback

The new copyFile implementation is based on uv_fs_copyfile(), and uses much faster native file copying api's, where available. See nodejs/node#14906 (comment).

[stefanpenner]

@kanongil good idea, especially now that node 8.5 is more utilized. I have a PR open -> #41

I don't want to land without thorough windows testing, so if you have some cycles I would love feedback on testing the PR

[stefanpenner]

@kanongil I'll need to test this again, last time I tried, it was not at all faster for smallish files.