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/doxygen_ignore.i

@@ -0,0 +1,41 @@
+%module doxygen_ignore
+
+%feature("doxygen:ignore:transferfull");
+%feature("doxygen:ignore:compileroptions", range="line");
+%feature("doxygen:ignore:forcpponly", range="end");
+
+#ifdef SWIGJAVA
+%feature("doxygen:ignore:beginJavaOnly", range="end:endJavaOnly", contents="parse");
+%feature("doxygen:ignore:beginPythonOnly", range="end:endPythonOnly");
+#elif defined(SWIGPYTHON)
+%feature("doxygen:ignore:beginJavaOnly", range="end:endJavaOnly");
+%feature("doxygen:ignore:beginPythonOnly", range="end:endPythonOnly", contents="parse");
+#else
+%feature("doxygen:ignore:beginJavaOnly", range="end:endJavaOnly");
+%feature("doxygen:ignore:beginPythonOnly", range="end:endPythonOnly");
+#endif
+
+%inline %{
+
+/**
+    A contrived example of ignoring too many commands in one comment.
+
+    @forcpponly
+    This is C++-specific.
+    @endforcpponly
+
+    @beginJavaOnly
+    This is specific to @e Java.
+    @endJavaOnly
+
+    @beginPythonOnly
+    This is specific to @b Python.
+    @endPythonOnly
+
+    @transferfull Command ignored, but anything here is still included.
+
+    @compileroptions This function must be compiled with /EHa when using MSVC.
+ */
+void func() { }
+
+%}