Update and fix kramdown settings and syntax highlighting

_config.yml

@@ -37,6 +37,22 @@ colors:
   yellow:        '#ffce3e'
   red:           '#de3c3c'
 
+  # More › http://kramdown.gettalong.org/quickref.html
+# Options › http://kramdown.gettalong.org/options.html
 kramdown:
   auto_ids:  false
+  input:          GFM
+  # https://github.com/jekyll/jekyll/pull/4090
+  syntax_highlighter: rouge
 
+  # Rouge Highlighter in Kramdown › http://kramdown.gettalong.org/syntax_highlighter/rouge.html
+  # span, block element options fall back to global
+  syntax_highlighter_opts:
+    # Rouge Options › https://github.com/jneen/rouge#full-options
+    css_class: 'highlight'
+    #line_numbers: true # bad idea, spans don't need linenos and would inherit this option
+    span:
+      line_numbers: false
+    block:
+      line_numbers: true
+      start_line: 1