[FEATURE] Add support for external links

External links may now be defined in Settings.yml:

------------------
conf.py:
  copyright: 2014
  project: Test
  version: 1.0
  release: 1.0.0
  extensions:
    - sphinx.ext.intersphinx
    - sphinx.ext.extlinks
  extlinks:
    forge:
    - https://forge.typo3.org/issues/%s
    - 'forge: '
    ter:
    - http://www.typo3.org/extensions/repository/view/%s
    - 'TER: '
------------------

and used with roles 'forge' and 'ter':

Bug has been reported to :forge:`58289`, extension on :ter:`sphinx`.

Additional documentation: http://sphinx-doc.org/ext/extlinks.html

Change-Id: Iea183e814732b9cf2ea7e16b3f0c90d01cd8fd57
Resolves: #58289
Reviewed-on: https://review.typo3.org/29709
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers