insert an ssd in graph when feedback connection detected

[michaelpalumbo]

as of #182 , Audio.js can detect feedback connections. however, it prevents passing them along to genish, which means that while we see the feedback cable in VR, we do not hear a self-patched sound.

• [ ] ~when feedback is detected, modify the patch.document to include a new ssd object, between the src and dest nodes.~
• [ ] ~confirm that if a feedback path has an ssd object, that it doesn't trigger a true from Audio.js:docHasFeedback()~
• [ ] ~set ssd cable colour to red~ Update: the modifications only need to take place in Audio.js, the VR scene doesn't need to see this.

[michaelpalumbo]

~since the feedback cable has an op that is only going to exist in the audio graph, should we create a new property in the patch.document for feedback objects? no, this complicated the doc2graph process in genish. instead, let's add it as a node in the graph and give it a property for not visualising it?~