Fixed incorrect options passing.

In *markdown.js* there was the following:

```javacript
grunt.registerMultiTask('markdown', 'Compiles markdown files into html.', function() {
  // Merge task-specific and/or target-specific options with these defaults.
  var options = this.options({
    htmlExtension: 'html',
    markdownExtension: 'md',
    markdownOptions: {},
    template: path.join(__dirname, 'template.html')
  });
  var template = grunt.file.read(options.template);

  // Iterate over all specified file groups.
  grunt.util.async.forEachLimit(this.files, 25, function (file, next) {
      convert(file.src, file.dest, next);
  }.bind(this), this.async());

  function convert(src, dest, next){
    var content = markdown.markdown(
      grunt.file.read(src),
      options.markdownOptions,
      template
    );

    grunt.file.write(dest, content);
    grunt.log.writeln('File "' + dest + '" created.');
    next();
  }
});
```

Where `options.markdownOptions` is passed to the *lib/markdown.js* module.

This is, of course, incorrect, since all the options, but **empty** `markdownOptions` are ignored and it should be `options` instead.