Changes for version 1.20
- add new ext_enable and ext_disable methods for selectively enabling or disabling only the specified subset of all supported extensions
- add new "ust" extention for united states territories. this extension is ALWAYS on unless you explicitly disable it, thus it retains compatibility with previous releases.
- rewrite ext method so that the extension list is constructed in a more straightforward fashion. while the behavior of the ext method was never fully documented, the behavior now is, for multiple flags for the same extension (ie, qw(usm -usm usm))), the most recent element in the list is the one that is honored.