Allow setting full_command so we can use custom functions again

With the current setup, we cannot call custom functions like `call SendToTmux(....)`, allow setting `g:rspec_full_command` so we can use something like

```vim
let g:rspec_full_command = 'call SendToTmux("rspec {spec}\n")
```

and workflow like mentioned in this blog post http://robots.thoughtbot.com/running-specs-from-vim-sent-to-tmux-via-tslime can work with minor changes again.