license_text grabs the actual plaintext of a set of
licenses within the provided metadata, wherever possible.
metadata returned from
further arguments to pass to httr's GET.
a data.frame of 3 columns; "license" containing the license IDs,
"content" containing the actual text, and "retrieved_from" indicating the
URL the content was returned from. In the event that the plaintext of a license
is not available (because a URL for it was not in the metadata),
retrieved_from will be NAs.