Add remove widget button

Summary: Added a 'remove' button on the header of the widget editor to quickly remove it from the question

Mostly an ice breaker for me as this is my first PR, so I used this little change (hopefully somewhat useful) as a way to get familiar with React and Perseus.

What has been done:
* Wrote a widgetPlaceholder string as to avoid repetition in the code
* Wrote a widgetRegExp string for the same reason (although it could be improved)
* Wrote a _handleWidgetEditorRemove function that removes the selected widget from the textarea content and calls re-rendering
* Placed a remove button in the header of the WidgetEditor
Move RegExp instantiation outside of the render function
Merge pull request #9 from AkA84/add-remove-widget-button

Add remove widget button