Add ability to specify substep order (#779)

The substep plugin currently shows each substep in the order in which
it appears in the HTML.  This is not always an ideal fit for some
presentation styles, where it would be helpful to specify a different
order (e.g., to add annotations to an image).

This commit allows users to specify a custom order via the
`data-substep-order` attribute.  Substeps without a
`data-substep-order` attribute are revealed last.

This commit also updates the Substep README to document the new
feature.

package-lock.json

@@ -1,6 +1,6 @@
 {
   "name": "impress.js",
-  "version": "1.0.0",
+  "version": "1.1.0",
   "lockfileVersion": 1,
   "requires": true,
   "dependencies": {