adding step-middle as interpolation method
change step-middle to just step
cleaner formatting for step interpolation method
Add brush.clamp method to specify to clamp extent to range or not
Make clamp implementation more similar to extent
Don't consider dragging when determining to clamp or not
Merge branch 'master' of github.com:jisaacks/d3 into 3.2
Merge branch 'add-step-middle' of github.com:iterion/d3 into 3.2