Open repository in the associated application by default

[siketyan]

ghr open, ghr init --open, and ghr clone --open requires specifying what application to open currently. When opening the repo in the associated application (i.e. Finder on macOS), we need to specify them as ghr open <repo> open or ghr clone <repo> --open open. It's a little complicated and not easy to understand.

So we need to open repository in the associated app by default, without specifying what app to open. i.e. ghr open <repo> opens the repo in the app.