Make generated float constant precision a command line option.

[kraiskil]

Number of decimals printed for float constants is hard-coded to 20 in:
https://github.com/kraiskil/onnx2c/blob/87a548021c4da69ed7bc0ce4c7c41eaae0249282/src/main.cc#L25

The number of decimal points should be a command line option (with 20 being a good default).