<extend> to use audio concat

[jtreminio]

When using <extend>, only the audio from the first video stage is kept.

This PR adds an "Audio Concat" node(s) to append each stage's audio.

To test:

A young woman, looking at the viewer, in a sunny park. She wears a bright, thin, yellow sundress. The sun shines behind her, making her dress semi-translucent.

<video>The young woman smiles, and says "Hello, welcome to my world"
<extend:161>The young woman frowns and says, "What are you looking at?"

Generates:

Adding more:

A young woman, looking at the viewer, in a sunny park. She wears a bright, thin, yellow sundress. The sun shines behind her, making her dress semi-translucent.

<video>The young woman smiles, and says "Hello, welcome to my world"
<extend:161>The young woman frowns and says, "What are you looking at?"
<extend:161>The young woman turns around then looks back over her shoulder at the viewer before smiling and winking and saying, "Haha just kidding, gooner!"

One unrelated question: vidChunks seems unused. Remove it?