Add Math Expression to markdown editor in Studio.

Provides possibility to add a Math Expression input from the common problem type editor ([BLD-486](https://edx-wiki.atlassian.net/browse/BLD-486)).
Screenshots are in the ticket.

For example:
```
= \(a + b\) 
```
should result in 
```xml
<formularesponse type="ci" samples="a,b@-10,-10:10,10#10" answer="a+b">
    <responseparam type="tolerance" default="0.00001"/> 
    <formulaequationinput size="40" />    
</formularesponse>
```
It should default to each variable having the range [-10,10] with ten samples, a tolerance of .000001 and input size of 40, as shown. If the user us unsatisfied with these default values, she can change them using the advanced editor.

@sarina, @singingwolfboy, @frrrances please review.