Add relaxed voicing assimilation as an option.

    `C' and `D' in suffixes like -CI, -DIk normally go through voicing
    assimilation after a unvoiced consonant to become ç and t
    respectively. However, this rule is violated rather often, such as
    `açıkca', `hoşnutmuşumcasına', `görüşdükten'... This change makes
    this it an option to relax the rule.
Merge branch 'master' into childes
Make RELAXED_C_ASSIMILATION default.