ENH: option to disable individual rules

[petrrr]

I have not found a way to disable individual rules, but think this would be a great feature.

In some cased StationXML documents would trigger on some rule, but the situation cannot be really resolved. In particular, the interpretation of the validator might be considered too strict, or simple not completely correct.

Especially for automized checking disabiling only some list of rules, would allow to keep checking against the other rules and get a clear result.