Add keybinding for ignoring directory under the cursor

[knuesel]

I often have directories that look like this:

   30.3 GiB [#######   ] /a                                                                                                           
    2.2 GiB [          ] /b                                                                                                          
    2.5 MiB [          ] /c                                                                                                      
   92.3 KiB [          ] /d
   ...

In such cases the graphical bars are not very useful...

I'd love being able to move to the first entry and press I (or something) to add it to the list of ignored directories.