Add the ability to search for related / similar modules

[mcdonnelldean]

We can augment search results with keywords or tokens to enable this. We need some discussion on what constitutes similar though.

Moved from: nodezoo/nodezoo-web#66

[matt-oc]

@mcdonnelldean There are keywords provided that come straight from npm itself returned in the npm module, could be of some use eventually?

"main": "lodash",
      "keywords": [
        "browser",
        "client",
        "functional",
        "performance",
        "server",
        "speed",
        "util"
      ],