Add docx as a renderer, set template on creation

This PR enhances the usability of the gem. 

It enables the configuration of custom and default templates and xslt files and it modifies the current document create method so that it can receive a custom template name that will be lookup in the custom_templates_path.

Also, inspired by the [axlsx_rails](https://github.com/straydogstudio/axlsx_rails) gem it adds the docx as a renderer, simplifying the use of the gem on rails apps. See README for detailed usage.