Added dragvisuals.vim

Thanks go to Damian Conway for his public-domain plugin.
Vim: Add 'vis' extensions plugin

Provides :B and :S commands to operate on selections.
Vim: hook up 'D' to duplicate selection
Use dragvisuals.vim for selection dragging.
Use Vim's builtin move command for visual mode.

Dragvisual's didn't support dragging a simple visual selection.

While we still can't drag left/right, it's at least possible to move the
lines including the selection.