Add namespace property

Hi,

I needed the ability to set a namespace on the output template, (much like grunt-contrib-jst: https://github.com/gruntjs/grunt-contrib-jst), so I added a small modification to do so.

I wasn't sure how to make this feature apply to multiple gulp input streams, so perhaps I will update once I figure that out.

I added two parts to test.js also.

Please let me know if there are additional changes that need to be made.

Thanks,

Brekk