Do not throw on unknown node

[sensiblearts]

I'm using Milkdown, which combines remark, for markdown, with ProseMirror.

Remark allows custom directives to add nodes, and I am adding <audio/> and <video/> tags using directive like:

::audio{src="/some/audio_file.wav"

Because of this, my docx export stops working when prosemirror-docx throws when encountering such nodes.

I don't totally understand your code base, but this change lets my export continue to work.

(As I learn more about your code, I'll try to add audio and video capability, if it can be done wisely.)