Changed --mathjax to link to mathjax CDN by default.

`--mathjax` now takes an optional URL argument. If it is not provided,
pandoc links directly to the (secure) mathjax CDN.
This is what they now recommend.  Thanks to dsanson.

README

@@ -238,9 +238,11 @@ Options
     no link to the jsMath load script will be inserted; it is then
     up to the author to provide such a link in the HTML template.
 
-`--mathjax=`*URL*
+`--mathjax`[=*URL*]
 :   Use [MathJax] to display embedded TeX math in HTML output.
     The *URL* should point to the `MathJax.js` load script.
+    If a *URL* is not provided, a link to the MathJax CDN will
+    be inserted.
 
 `--gladtex`
 :   Enclose TeX math in `<eq>` tags in HTML output.  These can then

src/pandoc.hs

@@ -288,8 +288,12 @@ options =
                  "" -- "Use jsMath for HTML math"
 
     , Option "" ["mathjax"]
-                 (ReqArg
-                  (\arg opt -> return opt { optHTMLMathMethod = MathJax arg})
+                 (OptArg
+                  (\arg opt -> do
+                      let url' = case arg of
+                                      Just u   -> u
+                                      Nothing  -> "https://d3eoax9i5htok0.cloudfront.net/mathjax/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML"
+                      return opt { optHTMLMathMethod = MathJax url'})
                   "URL")
                  "" -- "Use MathJax for HTML math"