Allow launching an external Python script on the CSV data

[wadetb]

To more easily access full NumPy (and other tools like MathPlotLib), allow a syntax =[range]!foo.py which serializes range (entire document if not specified), then launches a Python process to execute foo.py with the serialized data piped to stdin.

If the process outputs anything it's opened in a new "Evaluation results", or appended if that view already exists.

Need to check on the best serialization form, something that is easily translatable to a Python and/or NumPy array.