Introduce "iconv.translit" option.

For Russian language on Windows (under Cygwin) "Windows-1251" encoding
is used. LC_ALL environment variable is set to ru_RU.cp1251. For this
encoding Tig uses Iconv conversion from UTF-8 to translit (appends
//TRANSLIT//IGNORE to encoding set by LC_ALL). But for unknown reasons
conversion to this encoding gives corrupted strings in result (although
//TRANSLIT gives correct strings). So, let's introduce config variable
"iconv.translit" to be able to switch conversion applied.