addon support not optional when installed

We don't provide any configuration options for disabling support for any
of the third-party addons; they're only optional in the sense that
they're not required for installing vim-fish.  However, you can override
anything vim-fish does by configuring the other addons as needed.
don't overwrite existing snippets

The "!" option for UltiSnips snippets is meant for the user's own
snippets.  We want to allow the user to override our snippets without
having to care about the order of the snippet load path.
document &shell fix