Ticket 319: Make special treatment of CAPITAL SIGMA optional

Currently we require that if the characters GREEK LETTER CAPITAL SIGMA, SMALL SIGMA, and SMALL FINAL SIGMA are supported by an implementation, that a CAPITAL SIGMA in a string passed to string-downcase be changed to SMALL FINAL SIGMA just before a word break, and SMALL SIGMA otherwise. Word breaks are defined by UAX #29, and are no simple matter. I propose to make this behavior optional, allowing CAPITAL SIGMA to be downcased to SMALL SIGMA in every case.