Add SwapFilesWithPaneCommand

[kl0tl]

This allows to swap the currently selected file with files from another panes, nothing fancy but useful in some cases. I commented the keybindings because I use super+k, super+ctrl+shift+<direction> on my Mac but ctrl is already taken on Linux and Windows and I couldn’t think of any better combination.

[adzenith]

This looks good to me. The name made me think that it would swap all of the files in a pane with all of the other files in the other pane (essentially swapping panes). I'm trying to think if there's something more clear.

Would you be willing to add it to the Readme as well?

[adzenith]

Looking at the readme, I feel like we'd need to add a new section. Maybe split it into the basic commands and the more advanced commands. Could be a task for a different PR.

[kl0tl]

Should I rename the command to swap_current_files_with_pane or swap_active_files_with_pane ? It would be a bit more descriptive and would make things clearer if someone want a command that do what you thought, wouldn't it ? I did not needed such a command but I could implement one while I'm at it if you think it could be useful.

Will update the readme and maybe organize commands into sections when I'll find the time !