Added jumpnext and jumpprevious as commands

[SlaynAndKorpil]

When I was trying to change my keybindings for ncspot in config.toml, I noticed there was no way to change the 'vim like' jump-to-next-search-result command from their default keybindings "n" and "Shift+n".
This was especially annoying since I wanted to remove the default keybindings with default_keybindings = false, to only have my own keybindings which would mean losing these two keybindings.

This commit adds the "jumpnext" and "jumpprevious" commands which generate a Command::Jump(JumpMode::Next) and Command::Jump(JumpMode::Previous) to mimic the functionality of the default "n" and "Shift+n" keybindings.

This also makes it possible to use these as commands in the commandline.

[hrkfdn]

Thanks, merged! I added one small change to fix the serialization of jump commands as well :)

EDIT: Sorry, I messed up your merge branch 🙇