support secret blobs and add parseBlob

[mmckegg]

This PR adds support for detecting secret blobs using isBlob.

It also adds a new parseBlob export that when given a blob ref, returns an object containing {id, key}.

var ref = require('ssb-ref')
var secretBlob = '&abcdefg6bIh5dmyss7QH7uMrQxz3LKvgjer68we30aQ=.sha256?unbox=abcdefgqAYfzLrychmP5KchZ6JaLHyYv1aYOviDnSZk=.boxs'
ref.parseBlob(secretBlob) // => {
//    id: "&abcdefg6bIh5dmyss7QH7uMrQxz3LKvgjer68we30aQ=.sha256",
//    key: "abcdefgqAYfzLrychmP5KchZ6JaLHyYv1aYOviDnSZk="
//  }

I plan to use this in ssb-mentions and patchcore, rather than using slightly differing hand-rolled implementations that are currently there.

cc @clehner @dominictarr

[dominictarr]

to be honest, I think this is too specific. why not allow any query string on the end of an id?
same suggestion here: ssbc/ssb-mentions#11

[mmckegg]

Okay updated this module to match your proposal @dominictarr!

I'm gonna go ahead merge and publish this!

[mmckegg]

Released as v2.10.0 🎉

[mmckegg]

I unpublished 2.10.0 and instead submit #16