Add option to disable Blackfriday Smartypants

Can be used in site config or per page front matter:

```
[blackfriday]
smartypants = false
```

helpers/content.go

@@ -40,6 +40,7 @@ var SummaryDivider = []byte("<!--more-->")
 
 // Blackfriday holds configuration values for Blackfriday rendering.
 type Blackfriday struct {
+	Smartypants     bool
 	AngledQuotes    bool
 	Fractions       bool
 	HrefTargetBlank bool
@@ -52,6 +53,7 @@ type Blackfriday struct {
 // NewBlackfriday creates a new Blackfriday with some sane defaults.
 func NewBlackfriday() *Blackfriday {
 	return &Blackfriday{
+		Smartypants:     true,
 		AngledQuotes:    false,
 		Fractions:       true,
 		HrefTargetBlank: false,
@@ -148,9 +150,12 @@ func GetHTMLRenderer(defaultFlags int, ctx *RenderingContext) blackfriday.Render
 
 	htmlFlags := defaultFlags
 	htmlFlags |= blackfriday.HTML_USE_XHTML
-	htmlFlags |= blackfriday.HTML_USE_SMARTYPANTS
 	htmlFlags |= blackfriday.HTML_FOOTNOTE_RETURN_LINKS
 
+	if ctx.getConfig().Smartypants {
+		htmlFlags |= blackfriday.HTML_USE_SMARTYPANTS
+	}
+
 	if ctx.getConfig().AngledQuotes {
 		htmlFlags |= blackfriday.HTML_SMARTYPANTS_ANGLED_QUOTES
 	}