Adding an option to override the default SpecRunner template file

We had an issue whereby the requirejs code at the bottom of the outputted _SpecRunner.html file was including all of the jasmine.src files, that we actually didn't need to be included in the SpecRunner file, but we needed to specify them because https://github.com/maenu/grunt-template-jasmine-istanbul#templateoptionsfiles doesn't appear to instrument any of these files, only the ones in jasmine.src.

One bug aside, I can see use cases where users may want to customize their SpecRunner.html file