Add Functionality of asking bot about PR

[harryi3t]

A user should be able to ask the bot whether in a group or directly about the status of his PR.

Typical Syntax Would be

direct message to bot status PR
in a group @gitassist: status PR @username

[harryi3t]

@amalfra I think there should be a way to first let the bot know about which repository we are talking about, right? What do you say?
I mean something like @gitassist: set github url=github.com/user/repo
So that the first time the user enters
status PR our bot can reply
gitassist: No repo found. Please set by command 'set github url=<ur>'