Step mode

Makes use of https://github.com/code-dot-org/blockly-core/pull/67

This creates a new step mode for maze.  We introduce two new fields to the level.  When step is true, we will show a Step button to the right of the run button.  When stepOnly is true, we will not show the run button.

When you click the step button, we will gray out all of the blocks and go through the code one block at a time.  Resetting will bring you back to the same start state.  If you step until you get to the end of your code, you'll get the same experience as if you'd hit run.

I also added a couple of example levels with step functionality, though my dashboard PR will attempt to integrate this with level builder such that curriculum writers can come up with the proper progression themselves.