Correct initialisms as suggested by golint

First step to use initialisms that golint suggests,
for example:

    Line 116: func GetHtmlRenderer should be GetHTMLRenderer

as see on http://goreportcard.com/report/spf13/hugo

Thanks to @bep for the idea!

Note that command-line flags (cobra and pflag)
as well as struct fields like .BaseUrl and .Url
that are used in Go HTML templates need more work
to maintain backward-compatibility, and thus
are NOT yet dealt with in this commit.

First step in fixing #959.

hugolib/shortcode.go

@@ -203,7 +203,7 @@ func renderShortcode(sc shortcode, p *Page, t tpl.Template) string {
 		if sc.doMarkup {
 			newInner := helpers.RenderBytes(&helpers.RenderingContext{
 				Content: []byte(inner), PageFmt: p.guessMarkupType(),
-				DocumentId: p.UniqueId(), Config: p.getRenderingConfig()})
+				DocumentID: p.UniqueID(), Config: p.getRenderingConfig()})
 
 			// If the type is “unknown” or “markdown”, we assume the markdown
 			// generation has been performed. Given the input: `a line`, markdown