Implement audio filters

Use Source:setFilter(type, params...) to set a filter.
Use Source:setFilter() to detach the current filter.
Use Source:getFilter() to get the currently set filter (or nil).

Squashed from pull request 27, and the clean-pr27 branch in love-experiments.

--HG--
branch : minor

src/modules/audio/wrap_Source.h

@@ -23,6 +23,7 @@
 
 #include "common/runtime.h"
 #include "Source.h"
+#include "Filter.h"
 
 namespace love
 {