Adding an option to override the default SpecRunner template file

[davyboyhayes]

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

[amireh]

I've run into exactly the same issue, but I solved it by adding a skipSources option that prevents the template from including the sources in the require() call.

I can issue a PR if interested.

[Louisblack]

I know this is an old request but it gets another vote from me.
We're also having trouble with Istanbul coverage using this template.
We're also using Squire to mock some dependencies which causes the requirejs callback to fire multiple time so have needed to modify the template to deal with this.