Move local filesystem VFS implementation into a separate file and start to remove Node deps from Dropbox VFS impl

 * When building for the browser, avoid trying to load the Node 'fs'
   module.

 * In preparation for using Dropbox VFS in the browser, remove the 'fs'
   and Node auth server dependencies from it.
Add 'copy' command to CLI

Add interface for clipboard access, a fake implementation
and a 'copy' command to the CLI which copies fields
whose name, title or designation matches a given pattern
to the clipboard.
Linux clipboard implementation using xsel
Display 'Copied $FIELD from $ITEM to clipboard' only after copy completes

Avoid displaying the success message if the copy fails.
The error message on Linux is still unhelpful if xsel is not installed.
Add unit test for platform clipboard implementation
Use fake clipboard on Linux systems without an X server