Support choose preferred email address

[PeterDaveHello]

I have multiple email address but can not choose which one I want to use on submitgit.

[rtyley]

I'm closing this issue because I think it was addressed by #29 - specifically, patchbomb emails by SubmitGit are always sent 'from' the user's primary email address in GitHub.

[hramrach]

So you still cannot choose