Introduce a flexible way to tie in parameter constraints

[Dynom]

It's still quite trivial to abuse Imaginary with ideas like:

for width in $(seq 1 1000); do
  curl 'http://localhost:9000/?url=foo&width='${width};
done

If the result is proxied by a CDN, this results not only in a lot of computation time, but also quite a bit of storage costs. By adding a way to add constraints we can limit the amount of variations.

Some ideas:

• Limit to specific aspect-ratios
• Limit ranges 500 < width && width < 1000
• ...