allow returning to gVim by spawning terminal to background

[Konfekt]

To be able to return to gVim after spawning a terminal by the command line, append a & to the command. For example, :!urxvt & instead of :!urxvt.

However, currently the customization provided by vim-gtfo does not allow this. For example

let g:gtfo#terminals = { 'unix' : 'urxvt & -cd ' }

would always spawn to the current work dir. This relates to #25

[justinmk]

Oh, good point. Will fix