Change default selected tab from `Write` to `Code`

[jamestalmage]

Maybe when we've decided this is super stable, hide the Write tab, but for now I want to be able to break this extension without breaking GitHub.

[DrewML]

Yeah, I'm in favor of defaulting to this tab 👍 .

Having said that, we definitely can't hide the Write tab until we find some way to implement the @username auto-suggest that GitHub has. That's really the only reason I switch back to the Write tab, now.