Support the use of commonmark markdown via cmark-lua (#311)

Author: Shados

doc/doc.md

@@ -754,8 +754,11 @@ Niklas Frykholm. For convenience, LDoc comes with a copy of markdown.lua.
 more features than the pure Lua version, such as PHP-Extra style tables.
   - [lunamark](http://jgm.github.com/lunamark/), another pure Lua processor,  faster than
 markdown, and with extra features (`luarocks install lunamark`).
+  - commonmark via [cmark-lua](https://github.com/jgm/cmark-lua), a Lua wrapper
+around the fast [libcmark](https://github.com/jgm/cmark) C library (`luarocks
+install cmark`)
 
-You can request the processor you like with `format = 'markdown|discount|lunamark|plain|backticks'`, and
+You can request the processor you like with `format = 'markdown|discount|lunamark|commonmark|plain|backticks'`, and
 LDoc will attempt to use it.  If it can't find it, it will look for one of the other
 markdown processors; the original `markdown.lua` ships with LDoc, although it's slow
 for larger documents.

ldoc.lua

@@ -49,7 +49,7 @@ ldoc, a documentation generator for Lua, vs ]]..version..[[
   -l,--template (default !) directory for template (ldoc.ltp)
   -p,--project (default ldoc) project name
   -t,--title (default Reference) page title
-  -f,--format (default plain) formatting - can be markdown, discount or plain
+  -f,--format (default plain) formatting - can be markdown, discount, lunamark, commonmark, backticks, or plain
   -b,--package  (default .) top-level package basename (needed for module(...))
   -x,--ext (default html) output file extension
   -c,--config (default config.ld) configuration name

ldoc/markup.lua

@@ -275,6 +275,15 @@ local formatters =
                                                     { smart = true })
          return function(text) return parse(text) end
       end
+   end,
+   commonmark = function(format)
+     local ok, cmark = pcall(require, 'cmark')
+     if ok then
+       return function(text)
+         local doc = cmark.parse_document(text, string.len(text), cmark.OPT_DEFAULT)
+         return cmark.render_html(doc, cmark.OPT_DEFAULT)
+       end
+     end
    end
 }