Unsupported fzf_action ctrl-\ 

This configuration results in `unsupported key: ctrl-`
```vim
let g:fzf_action = {
      \ 'ctrl-t': 'tab split',
      \ 'ctrl-_': 'split',
      \ 'ctrl-\': 'vsplit',
      \ }
```
I think the issue can be connected with these values are not being escaped when passed to shell.