Add Hashtag Field to Supporters

[hagenburger]

As seen in #149, hashtags might be useful as an additional field for each event.

Suggested format:

{
  "usergroups": [
    {
      "name": "ThoughtWorks Tech Talks NYC Meetup",
      "city": "New York",
      "country": "USA",
      "link": "https://www.meetup.com/ThoughtWorks-Tech-Talks-NYC/",
      "hashtag": "ThoughtWorksNYC",
      "contacts": []
    }
  ]
}

Which links to https://twitter.com/search?q=%23ThoughtWorksNYC.