Allow registration of other translators

This adds an add_translator method to the sphinx application class.
It allows developers to register custom translators (writers) with
sphinx, so other extensions that add new nodes can register support
for the custom translators.

--HG--
extra : amend_source : 9d91784b449f6b54a0c4048b3cdae9abb8fc6517
Merged in russell/sphinx-pluggable-translators (pull request #229)

Allow registration of other translators