add option to leave properties unencrypted

[gr2m]

this can be used as a workaround for #13. It's a follow up to #18

db.crypto('password', {modp: 'modp5', ignore: '_attachments'})
.then(function () {
    return db.put({
      _id: 'id-12345678',
      _attachments: {
        'att.txt': {
          content_type: 'text/plain',
          data: 'TGVnZW5kYXJ5IGhlYXJ0cywgdGVhciB1cyBhbGwgYXBhcnQKTWFrZS' +
                'BvdXIgZW1vdGlvbnMgYmxlZWQsIGNyeWluZyBvdXQgaW4gbmVlZA=='
        }
      }
    });
  })

[gr2m]

I’ll clean up commits once I get the go from @calvinmetcalf

[calvinmetcalf]

again blocked by pouchdb-community/transform-pouch#19

[gr2m]

@calvinmetcalf could you explain please?

[calvinmetcalf]

I have a major refactor (#26 waiting on that pull)

[nolanlawson]

pouchdb-community/transform-pouch#19 is fixed now; anything else you need for this PR?

[gr2m]

closing in favor of #29