Create toNumericString function

Summary: If you can make this better, I'd appreciate any feedback (coding style as well as edge cases).

Test Plan:
```
Try out with ( . is undefined, * is all)
- Math.PI           pi / .          # π
- -Math.PI          pi / .          # -π
- Math.PI / 2       pi / .          # π/2
- -Math.PI / 3      pi / .          # -π/3
- -3*Math.PI/2      pi / .          # -3π/2
- .77               percent         # 77%
- -.77              percent         # -77%
- 3/2               mixed           # 1 1/2
- -7/2              mixed           # -3 1/2
- -.001             mixed           # -1/1000
- 1.5               improper / .    # 3/2
- -1.5              improper / .    # -3/2
- -.5               improper / .    # -1/2
- 5                 *               # 5
- 1                 *               # 1
- -1                *               # -1
- .0001             *               # 0.0001
- -.0001            *               # -0.0001
- Math.PI - .01     *               # 3.1315926535897933
```

Reviewers: jack

Reviewed By: jack

Subscribers: alpert

Differential Revision: http://phabricator.khanacademy.org/D7500