save knitting patterns to file

[niccokunzmann]

it should be possible to save, not only load the knitting patterns

• [ ] create JSONDumper
• [ ] convert knittingpattern to primitive structure
• [ ] do not add information that is in instructions by default

Hints:

• Image to knitting pattern conversion already creates a primitive structure.
• You may want to put the methods for doing this in the classes.