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.

sphinx/application.py

@@ -38,6 +38,11 @@ from sphinx.util import pycompat  # imported for side-effects
 from sphinx.util.tags import Tags
 from sphinx.util.osutil import ENOENT
 from sphinx.util.console import bold, lightgray, darkgray
+from sphinx.writers.html import HTMLTranslator
+from sphinx.writers.latex import LaTeXTranslator
+from sphinx.writers.text import TextTranslator
+from sphinx.writers.manpage import ManualPageTranslator
+from sphinx.writers.texinfo import TexinfoTranslator
 
 if hasattr(sys, 'intern'):
     intern = sys.intern
@@ -57,6 +62,15 @@ events = {
     'build-finished': 'exception',
 }
 
+TRANSLATORS = {
+    'html': HTMLTranslator,
+    'latex': LaTeXTranslator,
+    'text:': TextTranslator,
+    'man': ManualPageTranslator,
+    'texinfo': TexinfoTranslator
+}
+
+
 CONFIG_FILENAME = 'conf.py'
 ENV_PICKLE_FILENAME = 'environment.pickle'
 
@@ -440,6 +454,12 @@ class Sphinx(object):
             raise ExtensionError('Event %r already present' % name)
         self._events[name] = ''
 
+    def add_translator(self, name, translator_class):
+        if name in TRANSLATORS:
+            raise ExtensionError('A Translator by the name '
+                                 '%s is already registered.' % name)
+        TRANSLATORS[name] = translator_class
+
     def add_node(self, node, **kwds):
         self.debug('[app] adding node: %r', (node, kwds))
         nodes._add_node_class_names([node.__name__])
@@ -449,20 +469,9 @@ class Sphinx(object):
             except ValueError:
                 raise ExtensionError('Value for key %r must be a '
                                      '(visit, depart) function tuple' % key)
-            if key == 'html':
-                from sphinx.writers.html import HTMLTranslator as translator
-            elif key == 'latex':
-                from sphinx.writers.latex import LaTeXTranslator as translator
-            elif key == 'text':
-                from sphinx.writers.text import TextTranslator as translator
-            elif key == 'man':
-                from sphinx.writers.manpage import ManualPageTranslator \
-                    as translator
-            elif key == 'texinfo':
-                from sphinx.writers.texinfo import TexinfoTranslator \
-                    as translator
-            else:
-                # ignore invalid keys for compatibility
+            translator = TRANSLATORS.get(key)
+            # ignore invalid keys for compatibility
+            if not translator:
                 continue
             setattr(translator, 'visit_'+node.__name__, visit)
             if depart: