[qutebrowser] Run userscript in prompt mode

[Patrick]

Hi,

I'm currently trying to implement a basic password manager
userscript. I got it working like a charm for normal and 
input mode, but failed with prompt mode. 

When executing the userscript with a keybinding it seems to 
only run, after the prompt window closes. I also tried running 
the background script with `-d`, but without success. 

Is there currently any limitation on running userscripts in 
prompt mode? Can I even insert text from a userscript in 
prompt mode? I did read the official documentation on userscripts
but didn't see anything mentioned there (I might have missed it). 

Thanks.

Patrick.