Disable syntax guessing for PygmentsCodeFences by default

This disables highlighting for fenced code blocks without explicitly specified language. It also introduces a new `PygmentsCodeFencesGuessSyntax` config option (defaulting to false).

To enable syntax guessing again, add the following to your config file: `PygmentsCodeFencesGuessSyntax = true`

This is a breaking change.

content/overview/configuration.md

@@ -126,6 +126,8 @@ Following is a list of Hugo-defined variables that you can configure and their c
     preserveTaxonomyNames:      false
     # filesystem path to write files to
     publishdir:                 "public"
+    # enables syntax guessing for code fences without specified language
+    pygmentsCodeFencesGuessSyntax: false
     # color-codes for highlighting derived from this style
     pygmentsStyle:              "monokai"
     # true: use pygments-css or false: color-codes directly