Add the rest of the JSON tags to pass blogPosting Errors/Warnings

This adds the rest of the JSON fields to pass all errors and blog postings.

- Adds page.image.url for the image url. (Will default to image if not present). 
- Add page.image.height and page.image.width for an image object (Will default back to image url if not present). 
- Add dateModified (will capture from yaml if present, if not it will use datePublished)
  - (I feel there should be a manual option for this as I sometimes save parts of my blog that shouldn't update the modified field, but still 
change the file timestamp).

This should create JSON that will pass all warnings/strong recommendations/errors from [Google's Structured Data Testing Tool](https://search.google.com/structured-data/testing-tool).

lib/template.html

@@ -92,7 +92,7 @@
 {% endif %}
 
 {% if page.image %}
-  {% assign seo_page_image = page.image.path | default: page.image.facebook | default: page.image %}
+  {% assign seo_page_image = page.image.path | default: page.image.url | default: page.image.facebook | default: page.image %}
   {% unless seo_page_image contains "://" %}
     {% assign seo_page_image = seo_page_image | absolute_url %}
   {% endunless %}
@@ -214,13 +214,28 @@
 {% endif %}
 
 {% if seo_page_image %}
-    "image": {{ seo_page_image | jsonify }},
+    {% if page.image.height && page.image.width %}
+      "image": {
+          "@type": "ImageObject",
+          "url": {{ seo_page_image | jsonify }},
+          "height": {{ page.image.height | jsonify }},
+          "width": {{ page.image.width | jsonify }}
+      },
+    {% else %}
+        "image": {{ seo_page_image | jsonify }},
+    {% endif %}
 {% endif %}
 
 {% if page.date %}
     "datePublished": {{ page.date | date_to_xmlschema | jsonify }},
 {% endif %}
 
+{% if page.dateModified %}
+    "dateModified": {{ page.dateModified | date_to_xmlschema | jsonify }},
+{% elseif page.date %}
+    "dateModified": {{ page.date | date_to_xmlschema | jsonify }},
+{% endif %}
+
 {% if seo_description %}
     "description": {{ seo_description | jsonify }},
 {% endif %}
@@ -238,6 +253,13 @@
     },
 {% endif %}
 
+{% if seo_type == "BlogPosting" %}
+    "mainEntityOfPage": {
+         "@type": "WebPage",
+         "@id": {{ page.url | replace:'/index.html','/' | absolute_url | jsonify }}
+    },
+{% endif %}
+
 {% if seo_links %}
     "sameAs": {{ seo_links | jsonify }},
 {% endif %}