New PRs for matrix parameters for DID URLs

Based on feedback from the 2019-04-25 DID Spec and DID Resolution Spec
Weekly Meeting
<https://docs.google.com/document/d/1qYBaXQMUoB86Alquu7WBtWOxsS8SMhp1fioYKEGCabE/>,
I am replacing the initial PR for matrix parameters with new PRs that
separate 1. Introduction of the matrix parameter concept itself, and 2.
Concrete proposed matrix parameters. This way, we can better organize
the discussion and accept/change/reject the PRs separately.

Initial PR for matrix parameters (now closed):
https://github.com/w3c-ccg/did-spec/pull/187

New PR for matrix parameter concept, including new ABNF grammar,
excluding concrete matrix parameters:
https://github.com/w3c-ccg/did-spec/pull/189

Separate PRs for concrete proposed matrix parameters:
https://github.com/w3c-ccg/did-spec/pull/190 ("service")
https://github.com/w3c-ccg/did-spec/pull/191 ("service-type")
https://github.com/w3c-ccg/did-spec/pull/192 ("key")
https://github.com/w3c-ccg/did-spec/pull/193 ("key-type")
https://github.com/w3c-ccg/did-spec/pull/194 ("version-id" and
"version-time")
https://github.com/w3c-ccg/did-spec/pull/195 ("content-type" and
"content-id")
https://github.com/w3c-ccg/did-spec/pull/196 ("hl")

Markus