Add a command for opening the binary

[chasenlehara]

As a user, I’d like an easier way (out of the box) to open the compiled binary.

Right now, I have to refer to the guide to remember to type either open build/donejs-chat-darwin-x64/donejs-chat.app on macOS or .\build\donejs-chat-win32-x64\donejs-chat.exe on Windows.

It’d be nice if there was a command like donejs open electron