Beamer: Allow "noframenumbering" option (#4696)

As noted [here](https://tex.stackexchange.com/a/49805) ([beamer
commit here](ff70090f36)),
`noframenumbering` is an undocumented, but long existing option
to disable frame numbering for a particular slide. This is useful
to avoid numbering backup slides.

MANUAL.txt

@@ -4438,7 +4438,7 @@ introducing the slide:
 All of the other frame attributes described in Section 8.1 of
 the [Beamer User's Guide] may also be used: `allowdisplaybreaks`,
 `allowframebreaks`, `b`, `c`, `t`, `environment`, `label`, `plain`,
-`shrink`.
+`shrink`, `standout`, `noframenumbering`.
 
 Background in reveal.js
 -----------------------

src/Text/Pandoc/Writers/LaTeX.hs

@@ -402,7 +402,8 @@ elementToBeamer slideLevel  (Sec lvl _num (ident,classes,kvs) tit elts)
                     not (null $ query hasCodeBlock elts ++ query hasCode elts)
       let frameoptions = ["allowdisplaybreaks", "allowframebreaks", "fragile",
                           "b", "c", "t", "environment",
-                          "label", "plain", "shrink", "standout"]
+                          "label", "plain", "shrink", "standout",
+                          "noframenumbering"]
       let optionslist = ["fragile" | fragile && isNothing (lookup "fragile" kvs)] ++
                         [k | k <- classes, k `elem` frameoptions] ++
                         [k ++ "=" ++ v | (k,v) <- kvs, k `elem` frameoptions]