Add IsRenderable to Page

As pages are read from the target, they will be assessed if they should
be rendered or not.  The logic for IsRenderable is in the parser/page.go
and looks for anything exception '<'.

parser/page.go

@@ -39,6 +39,7 @@ type Content []byte
 type Page interface {
 	FrontMatter() FrontMatter
 	Content() Content
+	IsRenderable() bool
 }
 
 type page struct {
@@ -55,6 +56,10 @@ func (p *page) FrontMatter() FrontMatter {
 	return p.frontmatter
 }
 
+func (p *page) IsRenderable() bool {
+	return p.render
+}
+
 // ReadFrom reads the content from an io.Reader and constructs a page.
 func ReadFrom(r io.Reader) (p Page, err error) {
 	reader := bufio.NewReader(r)