Implement weights "pruning" logic

[Phil Pirozhkov]

By this point, we should have an extension point for custom pruning logic.

We need another class that implements weights based on pruning logic.

E.g. with:

# crystalball.yml
weights:
 - ["/app/models/*", 10]
 - ["/app/controllers/*", 5]
 - ["/app/mailers/*", 1]

Crystalball should fill examples limit to closest possible ratio as in weights.