Add grid step as an option to graphs

Summary:
Allow customizing the grid on a graph. If a grid step wasn't specified,
it calculates it in the way that it calculated it before. Uses
NumberInput instead of just a textbox like the other properties.

Test Plan:
* Load test.html and see that it starts with a reasonable value for the
  grid step
* Change test.html to have a grid step specified (just uncomment that
  part)
* See that it now uses the specified one instead of calculating one
  itself.
* Change the range to something small, then something large
* Note that like the 'step', the gridStep is adjusted to a reasonable
  value for the different ranges.

Reviewers: alex, jack

Reviewed By: jack

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D6233