Add `open` command

[amab8901]

Addresses https://github.com/rust-lang/mdBook/issues/1969.

This is intended to allow the user to browse the documents with the same convenience as mdbook build --open, but without building a book redundantly when it's already built. And without hijacking the terminal like mdbook watch and mdbook serve.

This is a rough first iteration. I have likely missed some details (I'm new to the codebase).