Add --open flag to cargo doc

Opens the generated docs in a browser.

Since it's more a utility feature, I didn't write this in a robust way. There's no error if it fails to open a browser.

For now I don't think this should pick up the `-o` flag, that's a pretty sweet flag to take for a fairly niche feature, and one day we may want to configure the output location of the documentation. If `--open` is super common, we can end up adding a short flag for it later