utils: keyword extraction

[jacquerie]

Just to prove that this is easy, so that we can demo it to curators next Wednesday.

Note the following neat Mustache trick:

{{#keywords.0}}
  <dt>Keywords</dt>
  <dd>
    <ul>
    {{#keywords}}
      <li>{{value}}</li>
    {{/keywords}}
    </ul>
  </dd>
  {{/keywords.0}

which only shows the line if there's at least one keyword (taken from http://stackoverflow.com/a/14417521/374865).

BTW, it appears that hep-0.0.1.json provides no way to represent keywords. I already asked @kaplun about it.

[jalavik]

:+1: Shall we merge?

[jacquerie]

:+1: for me, since @kaplun confirmed that there is no way to express free keywords in hep-0.0.1.json.

[kaplun]

We should add it :)