Adding support for extensions, functions that can assist with loading blocks, much like init functions, but that can be referenced from JSON definitions. This allows JSON definitions to define dynamic blocks such as onchange handlers and mutators.
Rewrote math_number as an example pure JSON block.
FieldNumber and FieldAngle previously accepted "undefined" as values, if not defined in JSON. This catches these and uses "0" for any NaN value. The constructor value parameter is now optional. Includes tests.
