Configurable ranges for interactive graphs.

Test Plan:
Add an interactive graph widget on test.html. Try altering
ranges, and see if the suggested steps are sensible. Make sure
bad range / step gives an error message instead of breaking. Try all
plot types with various ranges, including ranges with different scales.

Reviewers: alpert, emily

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D3110