Add support for GitHub-flavoured markdown code fences for highlighting

This commit adds a new PygmentsCodeFences config option (default false), which if true will allow GitHub style backtick code fences around code, which will then be rendered by Pygments.

For example:

``` language
your code
```

can be used instead of {{< highlight language >}}your code {{< /highlight >}}.

Fixes #362

commands/hugo.go

@@ -148,6 +148,7 @@ func LoadDefaultSettings() {
 	viper.SetDefault("PygmentsStyle", "monokai")
 	viper.SetDefault("DefaultExtension", "html")
 	viper.SetDefault("PygmentsUseClasses", false)
+	viper.SetDefault("PygmentsCodeFences", false)
 	viper.SetDefault("DisableLiveReload", false)
 	viper.SetDefault("PluralizeListTitles", true)
 	viper.SetDefault("PreserveTaxonomyNames", false)