Handle nouns with dual number

[psfinaki]

There are remainders of dual number in Czech. An example is the word ucho which has different forms for dual (uši) and plural (ucha). Earlier it was a common thing, now only a few nouns keep this phenomenon.

In terms of this project, a reasonable behavior would be to accept both dual and plural forms as an answer. Currently only the latter would be accepted - which is even somewhat confusing because people do not really consider forms like oka and ucha as plurals for related words.