Add cryptosuites to issuer config

[dlongley]

cryptosuites should be an array of cryptosuite options objects that will be used by the issuer. It should look like this:

cryptosuites: [{cryptosuite: 'cryptosuite name (same values as the old suiteName), options: {mandatoryPointers}}, {...}]

Support for suiteName should remain for backwards compatibility, but only one of cryptosuites or suiteName should be used.

[dlongley]

See #131 as well.

[aljones15]

Work on cryptosuites as an issuer config option was started here: https://github.com/digitalbazaar/bedrock-vc-issuer/tree/add-issuer-provision-option-cryptosuites

[dlongley]

I don't know why I wrote that we'd want this field to be an array, there's just one cryptosuite per issuer instance, so we can simplify that bit.

[dlongley]

This work is in progress now -- existing work is out of date and we need a slightly different direction. Also, the reason I wrote that we wanted an array was to support proof sets, so we do want it to function that way.

[dlongley]

This was done in #142, closing.