Add more parameters to the API

[krychu]

Examples include:

• Strategy to select next cell to collapse: Entropy, First available, MRV (minimum remaining values)
• Strategy to select tile in a cell: Input image frequency, LCV (least constraining value), uniform
• Place to start: random, center, corner
• Seed

wfc_overlapping already has quite a few parameters. It'd be a good opportunity to re-think the API.