Add a rule to redirect digital.cabinetoffice.gov.uk

The GDS blog has now transitioned to gds.blog.gov.uk, and we have
mappings to redirect the individual blog posts, but not for all of
the other pages such as:

  /tag/*
  /category/*
  /author/*
  /blog/*
  /*/feed
  /2013/04/11/ and all similar date-based paths

These should all exist on gds.blog.gov.uk at the same path, so it
makes sense to add a rule for this site.
Remove trailing whitespace
Merge pull request #35 from alphagov/add_rule_for_gds_blog

Add rule for GDS blog