Support lineAnchors (or line-anchors) in code blocks, for HTML.

MANUAL.txt

@@ -2008,6 +2008,12 @@ block above will appear as follows:
       </code>
     </pre>
 
+The `numberLines` (or `number-lines`) class will cause the lines
+of the code block to be numbered, starting with `1` or the value
+of the `startFrom` attribute.  The `lineAnchors` (or
+`line-anchors`) class will cause the lines to be clickable
+anchors in HTML output.
+
 A shortcut form can also be used for specifying the language of
 the code block:
 

src/Text/Pandoc/Highlighting.hs

@@ -85,6 +85,8 @@ highlight syntaxmap formatter (ident, classes, keyvals) rawCode =
   let firstNum = fromMaybe 1 (safeRead (fromMaybe "1" $ lookup "startFrom" keyvals))
       fmtOpts = defaultFormatOpts{
                   startNumber = firstNum,
+                  lineAnchors = any (`elem`
+                        ["line-anchors", "lineAnchors"]) classes,
                   numberLines = any (`elem`
                         ["number","numberLines", "number-lines"]) classes,
                   lineIdPrefix = if null ident