[Delivers #82528944] Add ability to embed Blockly blocks within multi/match content

This provides the ability to embed Blockly blocks within the question/prompt content of multi/match/textmatch level types.
To use, reference a level name followed by `.X_blocks`, where X is the type of block XML you wish to extract from the level (e.g., `Calc Circles of Eval .2.solution_blocks`), followed by an optional content-width value (using the same format as image references):

![image](https://cloud.githubusercontent.com/assets/56541/5193868/9f47103a-74ba-11e4-98a0-0ffb06e677b1.png)

This will embed the selected block XML within an iframe in the resulting level:

![image](https://cloud.githubusercontent.com/assets/56541/5193854/726d471e-74ba-11e4-8e55-3e431f3da449.png)
btw, is that ext.tap... thing intended to return the string starting at the second character? might be cleaner to do ext.slice(1..-1)