Re-organise command-line options for output file selection

[saxbophone]

Currently only one file can be specified for output. This keeps the program simple but means that the program can't be used to output a solved spiral to both sxp format and an image.

There a few different uses for reading and writing that sxbp has:

1. Reading in raw binary data to produce a spiral sequence from
2. Reading in a partially-complete or fully-complete serialised spiral object to solve it or render it to image
3. Writing out serialised spiral objects to file for later usage
4. Writing out images of spiral objects