Add a possibility to flexibly ignore custom Doxygen tags.

Add %feature("doxygen:ignore:<command>") implementation, documentation and
test case.

This feature allows to use custom tags in C++ Doxygen comments for
C++-specific things that don't make sense in the context of the target
language and also allows to insert contents specific to the target language in
the C++ comments using (different) custom commands, which is very useful in
practice to explain the particularities of the API wrappers.

Examples/test-suite/python/doxygen_ignore_runme.py

@@ -0,0 +1,21 @@
+#!/usr/bin/python
+
+import doxygen_ignore
+import commentVerifier
+
+commentVerifier.check(doxygen_ignore.func.__doc__,
+    r"""
+    A contrived example of ignoring too many commands in one comment.
+
+
+
+
+
+
+    This is specific to **Python**.
+
+
+    Command ignored, but anything here is still included.
+
+
+    """)