Support variable, error penalties by error type (weighted errors)

[dylon]

Currently, each error type (insertion, deletion, substitution, transposition, merge, and split) incurs a penalty of 1 unit. Allowing the penalties to be variable (e.g. making deletions worth 3 units, while leaving the others at 1 unit) will allow finer-grained control over the search strategy for spelling candidates. This will be a particularly-useful feature for various, non-English languages.