Template/partial/line_number traces on generated DOM elements 

Adding a trace to a DOM element data tag containing the path of the partial and line number of the location where the element is coming (rendered) from in a haml template. This feature proved useful for a new developer starting out on a complex project that uses haml templates.
To enable it, add this configuration:

  Haml::Template.options[:show_partial_trace] = true

note: I tried to see how i would go about testing it, but could not easily figure it out. Would appreciate if the maintainer can help covering this with appropriate tests.

Thanks!