Measurer: Add width/height option

Summary: Adds a width and height option to control the size of a measurer box

Test Plan: http://localhost:9000/test.html#content=%7B%22question%22%3A%7B%22content%22%3A%22**Measure%20the%20length%20of%20each%20line%20to%20the%20nearest%20%24%5C%5Cdfrac%7B1%7D%7B4%7D%24%20cm%2C%20and%20then%20plot%20the%20measurements%20of%20the%20lines%20on%20this%20line%20plot.**%20%20%20%5Cn%5B%5B%E2%98%83%20measurer%201%5D%5D%5Cn%5Cn%5B%5B%E2%98%83%20plotter%201%5D%5D%5Cn%22%2C%22widgets%22%3A%7B%22measurer%201%22%3A%7B%22type%22%3A%22measurer%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22imageUrl%22%3A%22https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2Fa74adc9e608c2ecb03551040f59bfe97961d2ca2.png%22%2C%22imageTop%22%3A0%2C%22imageLeft%22%3A0%2C%22width%22%3A480%2C%22height%22%3A200%2C%22showProtractor%22%3Afalse%2C%22showRuler%22%3Atrue%2C%22rulerLabel%22%3A%22cm%22%2C%22rulerTicks%22%3A4%2C%22rulerPixels%22%3A40%2C%22rulerLength%22%3A10%7D%7D%2C%22plotter%201%22%3A%7B%22type%22%3A%22plotter%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22correct%22%3A%5B0%2C0%2C0%2C1%2C0%2C0%2C0%2C0%2C0%2C0%2C0%2C0%2C1%2C0%2C0%2C0%2C0%2C1%2C0%2C0%2C0%5D%2C%22starting%22%3A%5B0%2C0%2C0%2C0%2C0%2C0%2C0%2C0%2C0%2C0%2C0%2C0%2C0%2C0%2C0%2C0%2C0%2C0%2C0%2C0%2C0%5D%2C%22type%22%3A%22dotplot%22%2C%22labels%22%3A%5B%22Length%20(cm)%22%2C%22%22%5D%2C%22categories%22%3A%5B%22%242%24%22%2C%22%24%24%22%2C%22%24%24%22%2C%22%24%24%22%2C%22%243%24%22%2C%22%24%24%22%2C%22%24%24%22%2C%22%24%24%22%2C%22%244%24%22%2C%22%24%24%22%2C%22%24%24%22%2C%22%24%24%22%2C%22%245%24%22%2C%22%24%24%22%2C%22%24%24%22%2C%22%24%24%22%2C%22%246%24%22%2C%22%24%24%22%2C%22%24%24%22%2C%22%24%24%22%2C%22%247%24%22%5D%2C%22scaleY%22%3A1%2C%22maxY%22%3A10%2C%22snapsPerLine%22%3A2%2C%22labelInterval%22%3A1%7D%7D%7D%7D%2C%22answerArea%22%3A%7B%22type%22%3A%22multiple%22%2C%22options%22%3A%7B%22content%22%3A%22Measure%20the%20length%20of%20each%20line%2C%20and%20create%20a%20line%20plot%20that%20shows%20the%20measurements.%22%2C%22widgets%22%3A%7B%7D%7D%2C%22calculator%22%3Afalse%7D%2C%22hints%22%3A%5B%7B%22content%22%3A%22Line%20up%20the%20ruler%20to%20each%20line%20so%20the%20edge%20of%20the%20ruler%20is%20lined%20up%20with%20the%20end%20of%20the%20line.%5Cn%5Cn!%5B%5D(https%3A%2F%2Fka-perseus-images.s3.amazonaws.com%2F77752e5a8dd9992549121dbf8c70c40482a839fd.png)%5Cn%5CnThis%20line%20measures%20%24%5C%5Cpurple%7B6%20%5C%5Cdfrac%7B1%7D%7B4%7D%5C%5Ctext%20%7B%20cm%7D%7D%24.%20%20%20%22%2C%22widgets%22%3A%7B%7D%7D%2C%7B%22content%22%3A%22Place%20a%20dot%20above%20%24%5C%5Cpurple%7B6%20%5C%5Cdfrac%7B1%7D%7B4%7D%5C%5Ctext%20%7B%20cm%7D%7D%24%20on%20the%20number%20line.%20%20%5Cn%5Cn!%5B%5D(https%3A%2F%2Fka-perseus-images.s3.amazonaws.com%2F61fdaea2f905e4715b55611b5eacc9c5126d5cf2.png)%22%2C%22widgets%22%3A%7B%7D%7D%2C%7B%22content%22%3A%22The%20measures%20of%20all%20%243%24%20lines%20are%3A%20%20%5Cn%24%5C%5Cqquad%24%24%5C%5Cgreen%7B5%20%5C%5Ctext%20%7B%20cm%7D%7D%24%2C%20%24%5C%5Cred%7B2%5C%5Cdfrac%7B3%7D%7B4%7D%20%5C%5Ctext%20%7B%20cm%7D%7D%24%2C%20%24%5C%5Cpurple%7B6%20%5C%5Cdfrac%7B1%7D%7B4%7D%5C%5Ctext%20%7B%20cm%7D%7D%24%20%20%22%2C%22widgets%22%3A%7B%7D%7D%2C%7B%22content%22%3A%22!%5B%5D(https%3A%2F%2Fka-perseus-images.s3.amazonaws.com%2Fef67984b757fa869d487f65a222321fb65afa5ea.png)%5Cn%22%2C%22widgets%22%3A%7B%7D%7D%5D%7D

Reviewers: justinjaffray

Reviewed By: justinjaffray

Subscribers: alpert, alex

Differential Revision: http://phabricator.khanacademy.org/D7778