Added view_path option

I've created a [grape-jbuilder gem](http://github.com/milkcocoa/grape-jbuilder) for using Jbuilder in Grape.
This gem uses this tilt-jbuilder gem, but to support Jbuilder partial! method, I needed I had tilt-jbuidler supported view's root path.
So, I introduced a view_path option in tilt-jbuilder.