Feature Request: infer target format by extension if --to option is omitted

[romeokienzler]

I'm running the following command:
jupyter nbconvert WFMTutorial.ipynb WFMTutorial.py

As expected, this is the error message:

`The following formats are available: ['asciidoc', 'custom', 'html', 'latex', 'markdown', 'notebook', 'pdf', 'python', 'qtpdf', 'qtpng', 'rst', 'script', 'slides', 'webpdf']

It would be nice of in the above case semantically the following command would be inferred:

jupyter nbconvert WFMTutorial.ipynb --to script

I think it would work for most of the formats
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