Matrix parameters for DID URLs

After various conversations e.g. on recent DID Spec and DID Resolution
calls, there's now a proposal
<https://docs.google.com/document/d/1TctFY8euBH2wq7Z8c9KccICDZUGZplvhoqlHlFMahGk/>
to use "matrix parameters" in DID URL syntax.
There's also a second document
<https://docs.google.com/document/d/1VpYPvUw2o-01e727bCy2V-0MUEe0_NB-EqX677scQbI/>
that lists use cases for such parameters.

That second document also compares other options for expressing
resolution parameters (using colons instead of semicolons).

Matrix parameters were first proposed
<https://www.w3.org/DesignIssues/MatrixURIs.html> in 1996.

Please review and post comments, and let's see if we have consensus to
adopt a DID ABNF (excuse me, a DID grammar) based on this proposal.

If you have time, please join tomorrow's combined DID Spec and DID
Resolution Spec call
<https://docs.google.com/document/d/1qYBaXQMUoB86Alquu7WBtWOxsS8SMhp1fioYKEGCabE/>
where we will discuss this.

Markus and =Drummond