Add custom keybinds to config

[LucasPickering]

This allows you to override the default keybindings. For example, vim bindings:

input_bindings:
  up: [k]
  down: [j]
  left: [h]
  right: [l]
  scroll_left: [shift h]
  scroll_right: [shift l]

Closes #137

[LucasPickering]

@quantonganh This should allow you to do vim keybindings. If you could test this out and make sure it works for you I'd appreciate it. You can install it like so:

cargo install --git https://github.com/LucasPickering/slumber --branch custom-keybinds

Depending on how you originally installed slumber, your original version may override that one in PATH. If it's not working do which slumber to make sure it's using the correct version.

[quantonganh]

@LucasPickering Thank you for implementing this. It works as expected.

To anyone who also want to use vim key bindings, don't forget to bind the select_recipe_list to another key (I use e which stands for endpoints) to make it not conflict with the right action.

[LucasPickering]

@quantonganh Thanks for noticing that, I just updated the docs to rebind select_recipe_list as well. I'll merge this and release today.