Add -L option as shortcut for --lua-filter.

MANUAL.txt

@@ -549,7 +549,7 @@ Reader options {.options}
     Filters and lua-filters are applied in the order specified
     on the command line.
 
-`--lua-filter=`*SCRIPT*
+`-L` *SCRIPT*, `--lua-filter=`*SCRIPT*
 
 :   Transform the document in a similar fashion as JSON filters (see
     `--filter`), but use pandoc's build-in lua filtering system.  The given

src/Text/Pandoc/App/CommandLineOptions.hs

@@ -428,7 +428,7 @@ options =
                   "PROGRAM")
                  "" -- "External JSON filter"
 
-    , Option "" ["lua-filter"]
+    , Option "L" ["lua-filter"]
                  (ReqArg
                   (\arg opt -> return opt { optFilters =
                                     LuaFilter (normalizePath arg) :