Vim: try to center on :cn, :cnf etc

Based on the technique described in:

http://vim.wikia.com/wiki/Replace_a_builtin_command_using_cabbrev

This seems to work pretty well, although it would be nice if we could
make the patching invisible. As it is, you can see the `:cn | normal zz`
echoed to the command line.

roles/dotfiles/files/.vim/plugin/abbrev.vim

@@ -0,0 +1,5 @@
+" Keep quickfix result centered, if possible, when jumping from result to result.
+cabbrev <expr> cn ((getcmdtype() == ':' && getcmdpos() == 3) ? 'cn <bar> normal zz<cr>' : 'cn')
+cabbrev <expr> cnf ((getcmdtype() == ':' && getcmdpos() == 4) ? 'cnf <bar> normal zz<cr>' : 'cnf')
+cabbrev <expr> cp ((getcmdtype() == ':' && getcmdpos() == 3) ? 'cp <bar> normal zz<cr>' : 'cp')
+cabbrev <expr> cpf ((getcmdtype() == ':' && getcmdpos() == 4) ? 'cpf <bar> normal zz<cr>' : 'cpf')