Improvements on extensions

• Rename "converter" to "extension"
• Change/improve how extensions are defined in Python and Python-lite
• Test extension api of Python
• Make ndarray a standard extension (implement in all implementations)
• Allow custom converters in JS, and include standard converters.
• Add ndarray standard converter to Matlab impl.

Fixes #10 (closed), fixes #5 (closed), checks some boxes in #7 (closed).