Add a script for some stats on GOV.UK content

Known issues:
  * it only looks at the indexable_content field, so ignores titles in many or
    all cases. Lots of government content has no indexable_content.
  * the dictionary size is likely inflated because punctuation isn't stripped
Merge pull request #116 from alphagov/search_index_stats

Add a script for some stats on GOV.UK content