URL base for tag archives now specified in _config.yml

- The tag generation plugin hard coded the url base for tag archives to
  `/tag/{{tag}}`. Now we pull out of `_config.yml` the value for key
`tag_dir` and define the url base from there.
Fixes 39c2107

- I swear 39c2107 was working but this should fix the build broken by that
  commit
Merge pull request #181 from 18F/tags_dir-in-config

URL base for tag archives now specified in _config.yml