set up support for diff sample file based on domain people come in on

[rahulbot]

Related to #370

[rahulbot]

@kanarinka I've added a shortcut that lets us limit samples by domain. If you simple add a domains property to the sample it will be respected. For instance, this would let us limit this speeches entry to just the (fictional) "political.databasic.io" domain:

  {
    "title": "Hillary Clinton's Speeches",
    "source": "sample-data/en/hillary-clinton-speech.txt",
    "modules": ["samediff", "wordcounter"],
    "lang": "en",
    "biography": "A collection of speeches made by Hillary Clinton during the 2016 US presidential election campaign.  This isn't a comprehensive set, but does contain many of her speeches.  It was created manually from their website, closed captions from YouTube videos, and other sources by Catherine D'Ignazio in early 2016.",
    "domains": ["political.databasic.io"]
  },

Notes:

• If the sample has no domains entry (ie. all of them right ow), it just shows up everywhere.
• If the sample has an empty domains property (ie. "domains": []), it shows up nowhere.