bandcamp: Add `export-band` and `export-album` commands

This will enable us to make web-based playlists from content on Bandcamp, in as much as I hope to generate web pages that use the Bandcamp embedded player similarly to Bandcamp Daily pages.

The implementation uses some web scraping, so a dependency on BeautifulSoup4 library is added.

Done:

• Automated tests, using saved web pages
• Figure out why cpe -v 3 bandcamp export-band https://ztapes.bandcamp.com/ --expand-albums fails with:

urllib3.connectionpool https://bandcamp.com:443 "GET /EmbeddedPlayer/album=track-3884234695 HTTP/1.1" 200 None
TypeError: 'NoneType' object is not subscriptable

• Figure out caching: currently, if a band you exported already releases a new album, you won't see it as data will always be returned from the cache. We should add a timeout for caching, and let user control it as needed. Done in !195 (merged)

Fixes #97 (closed)