Allow multiple simultaneous BOOK_IDs and/or playlist ID

[detvdl]

Adds functionality to specify multiple book ids at once to be downloaded sequentially, as well as the possibility to specify a playlist id, to download all books gathered in the playlist.

My apologies for the many whitespace-related edits, but I have an editor plugin that automatically cleans up trailing whitespace.

[timblaktu]

@detvdl this is a useful feature, thanks for contributing. Can you describe how this has been tested? You might get more traction upstream if this is shown.

FWIW, I haven't tried your code yet, but I'm doing essentially the same thing in the shell outside safaribooks.py:

# Fetch all bookids from all playlists
curl -H 'Accept: application/json' https://learning.oreilly.com/api/v2/collections > collections.json
OREILLY_BOOK_IDS="$(jq -r '.[].content[].ourn | select(. | contains("urn:orm:book")) | sub("urn:orm:(?<type>[a-zA-Z0-9-]+):(?<id>[0-9]+)"; "\(.id)"; "g")' collections.json)"
for id in ${OREILLY_BOOK_IDS}; do
    echo "Fetching $id..."
    python safaribooks.py "$id"
done