signing with purpose via :for option

Work on #8 

There're many `options` everywhere :sweat_smile: But it seems hard to avoid them..

But I have some questions, so I hope to get some of your help :smiley:  @jeremy .I'll comment on the code
This method is expected to return `nil` when the Global ID couldn't be verified, too. So let's skip the new exception for now and just return the `gid if <purpose matches?>`