Add functionality of asking bot about whos working on what

[harryi3t]

A user should be able to ask the bot whether in a group or directly about whos working on what issue in the team.

Typical Syntax Would be

direct message to bot status team
in a group @gitassist: status team

Bot will reply
gitassist: m/n members are working on issues
member 1 : #123
member 2: #234
..
member m: #432

1. Call https://api.github.com/repos/harryi3t/slackassist/issues?state=open&per_page=100&page=1
   if no. of issues < 100 then increment page
2. the response is array of objects. We will use html_url, title and assignee.

Depends on #34