feat: allow for toggling of gitignored files

[fmgornick]

Hi! i'm a big fan of this plugin and thought this feature would be a nice addition, so i figured it'd be a good opportunity to try and contribute!

I saw there was a recipe provided for hiding gitignored files in an oil buffer and found that it was almost what i wanted. I personally prefer seeing them by default, but then toggling them off when they get in the way, so i thought it'd be nice if there was a toggle option similar to how toggle_hidden() works.

this change adds the ability to toggle on and off gitignored files with the default keymap gI (this command does nothing if not in a git repo btw). I'm not partial to gI or anything tho, so if you have a better idea for a default keymap i can always change it!

I also updated the docs accordingly and removed the aforementioned recipe since it's now a config option. I totally understand if this change seems too invasive, so no hard feelings if you wanna close it out!

[stevearc]

I think I'd prefer to keep this as a recipe that lives in the user config. I'd like the built-in logic to be pretty simple so it's easy to reason about adding customizations on top of it

[fmgornick]

makes sense!