Add a short pause between headers within an article and image captions.

[HugoRomano]

Right now when Polly is reading headers in an article, it feels like it needs a short pause after starting with the body again. Similar after image captions.

Examples:

1. No pause following image captions https://www.nytimes.com/2018/09/26/arts/color-factory-museum-of-ice-cream-rose-mansion-29rooms-candytopia.html
2. No pause after section headers: https://medium.com/dropbox-design/how-to-stay-scrappy-9c42af21c996

This is a lower priority item.