[wip] Added support to screen foreground, fixes #83.

Im trying to add support to screen foreground execution using splits. This is work done until now, but getting some errors, like:

```
:Dispatch sleep 2
```

Running this open the split but this errors appears on vim:

```
Error detected while processing function dispatch#compile_command..<SNR>57_dispatch..dispatch#screen#handle..dispatch#screen#spawn:
line   22:
E484: Can't open file /tmp/vu7hFCf/2
```

The file `/tmp/vu7hFCf/2` is the file on the `request.file` argument to `dispatch#screen#handle()` call.