Add the option to pass parameters in 'key' and 'component' values for mustache_string_helper

mustache_parameter_helper could receive 3 different parameters to call get_string function: $key, $component and $a as json format information. $a parameter could contain complex information that could include parameter or information for other helpers.

Adding that option to pass complex information to $key and $component parameters could make more simple the information passed to the mustache template, because would be easier to reuse some of the data sent.

This issue has been created because once this is integrated mustache created in MDL-67060 or MDL-67508, for example, could be easier.