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
Good point, but this method already properly handles `options` on master. Once this branch is rebased agains latest changes this line will no longer be part of the diff.