Support markdown in book descriptions

[MxMarx]

It would be pretty nice to have a little rich text in book descriptions, mostly so line breaks are preserved when fetching metadata.
This pull request uses turndown to convert book descriptions grabbed from Audible or iTunes into markdown, and since marked is already included to render the changelog, rendering markdown in the descriptions is pretty easy.

Alternatively, we could use the trix editor currently used for podcast episodes instead of using markdown. If this is something planned for the future, it might make more sense to store rich text descriptions as sanitized html instead of markdown. I could also make a pull request with the trix editor instead if wanted.

resolves #617
resolves #1820

[advplyr]

One issue I had with implementing this is descriptions are also embedded in audio file meta tags with the embed tool. I'm not sure if we want to be embedding markdown or HTML in meta tags.

If we keep this as a single description field then it makes more sense to me to use HTML since both Audible and iTunes use HTML.