When building a user dict, check number of fields

[polm]

Originally brought up in octanove/janlpbook-code#1.

When building a user dictionary, it would be helpful to check the number of fields and warn or something when they don't match the system dictionary. This is a little tricky because MeCab doesn't actually parse any dictionary fields, so it's valid to just omit everything, for example. This is further complicated since dictionaries don't have explicit schemas. So this would require a bit of making assumptions and departing from the MeCab API, but it would help make more useful user dictionaries.