Add support for doxygen:alias feature

This allows to replace non-standard Doxygen commands with some (fixed) text
instead of just ignoring them, as was already possible with the feature
doxygen:ignore.

Examples/test-suite/python/doxygen_alias_runme.py

@@ -0,0 +1,12 @@
+#!/usr/bin/python
+
+import doxygen_alias
+import inspect
+import commentVerifier
+
+commentVerifier.check(inspect.getdoc(doxygen_alias.make_something),
+    """\
+A function returning something.
+
+:rtype: :py:class:`Something`
+:return: A new object which may be None.""")