Add Page.RawContent() to access raw Markdown as a string.

In particular, RawContent() excludes the metadata header.

This is necessary in the use case of embedding remarkjs.com slides, as it needs
the unprocessed Markdown content to generate the slides.

hugolib/page.go

@@ -726,6 +726,10 @@ func (p *Page) parse(reader io.Reader) error {
 	return nil
 }
 
+func (p *Page) RawContent() string {
+	return string(p.rawContent)
+}
+
 func (p *Page) SetSourceContent(content []byte) {
 	p.Source.Content = content
 }