Retrieve composer information from The Session

Raised by @linneakirby in discussion of issue #412.

On The Session's website, the composer of the tune is sometimes listed on the tune page (e.g. "Ross Ainslie" is the composer of https://thesession.org/tunes/23650). However, this information is nowhere to be found in the [public datadump](https://github.com/adactio/TheSession-data).

We should consider scraping the Session's website to retrieve composer information.

This situation is different from the one involving The Session's "artists" (e.g. performers on an album), whose names are at least present in the dataset, despite not being attributed an URI (see issue #250).