Run user entered input as a command (see #16)

Dmenu for X Server runs whatever input you put in as a command even if it doesn't see an application; for example:

```dash
$ ls ~ | grep mp4
test.mp4
```

``` dmenu-mac
$ mpv test.mp4
```

Should open test.mp4 in mpv. Current behaviour: the prompt is not accepted; dmenu-mac shows "no results found"

See slightly similar #16 
