Helper updates for Tag Pages

fixes #2112
refs #1833

- modified config.urlFor to handle tag pages

- modified {{#tags}} handlebars helper to autolink to
  tag pages. Additional autolink="false" parameter can
  be used to deactivate autolinking

- modified url handlebars helper to handle tags

- added isTag function to schema

- added unit test for additional urlFor functionality

- added unit test for {{#tags}} helper modifications

- added unit test for url handlebards helper