Add option to configure icon pack -> icon delimiter

[mkusz]

Currently, when icons are added to a new icon pack, icon name looks like this: :icon-pack-name_icon-name:. Delimiter is an underscore _. If you want to publish a note using Material for MkDocs the delimiter used by it for icons is -. It would be great to have an ability to configure it and use the same icons as Material for MkDocs, so Obsidian can be used as a document editor.