Add output buffering for Lua pages

An empty table named `mg.buffers` is provided. Whenever `mg.buffers` is not empty and the last item is a string, bytes that would be sent to `mg_write` are instead concatenated to the last item in `mg.buffers`.

The "push" and "pop" functions are up to the user to implement in Lua, like this:

    function buffer_push()
      table.insert(mg.buffers, "")
    end

    function buffer_pop()
      return table.remove(mg.buffers)
    end

Google group thread [here](https://groups.google.com/forum/#!topic/mongoose-users/RCgAQFxYAmM).