Initial commit (extract from jsonld-signatures).

[dmitrizagidulin]

Depends on:

• Extract signature suites to separate repos. jsonld-signatures#126

Enables:

• Initial commit (extract from jsonld-signatures). ed25519-signature-2018#1

[dlongley]

Approving conditionally, see comments. Thanks!

[dlongley]

Note: Extending suites will need to check for their contexts here and in sign. We probably want a note about that somewhere.

[dmitrizagidulin]

I added a note - does that work?

[davidlehn]

CHANGELOG needs updating for name and/or other entries.