Add support for printf and houdini padding formats

[justinfx]

Need to extend the support of padding formats to include printf %04d and houdini $F4d to align with the Python fileseq library:
https://github.com/justinfx/fileseq/blob/1d6fa52f12952c8f5a54264875f009c6f83eda48/src/fileseq/constants.py#L43

Currently the C++ library will crash if a user tries to create a sequence with a printf pad syntax and then access frame values:

    std::string fileName = "dir/file.%04d.ext";
    fileseq::Status stat;
    fileseq::FileSequence sequence = fileseq::findSequenceOnDisk( fileName.asChar(), &stat );
    assert(bool(stat));
    // Crashes here
    std::string path = sequence.frame(frame);

Untested as to what happens in the Go library.