feat: relative numbers

[gbaranski]

closes #483

[gbaranski]

I'm not sure how I can pass misc configuration to render_buffer, do you have any ideas? render_buffer member of EditorView struct, which has keymaps field, maybe we could replace it with config: Config which has config.keymaps, or add another field called misc_config: MiscConfig?

[archseer]

I was thinking that maybe the config type could be moved to helix-view then placed onto editor.config. That way it's not term specific and you can access it from anywhere you have access to the Editor.

Themes already work that way for example.

[pickfire]

I think we could put part of the config within editor, I don't think we should have a single config for everything unless we are thinking of live patching config like alacritty.

[archseer]

I've added an [editor] config section that gets passed into Editor: a2ccfff should be enough to unblock this PR.