Add open command

[amab8901]

Addresses #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).

[mgeisler]

And without hijacking the terminal like mdbook watch and mdbook serve.

I asked on #1969, but maybe it's better here: why how just open the generated files directly? You have them right there on your disk, so they're only a

your-browser book/index.html

away 🙂

[amab8901]

I think it was mostly a minor issue. I don't remember all the details and it was a while ago. Let's close it for now. If someone else encounters this issue, then can file an issue report and PR and whatnot.