#37 Support for XeTeX

[Joachim Schrod]

Add XeTeX to the -I option values of texindy.
Document its usage.

It must imply the codepage utf8 and must not load an
inputenc module.

I.e., it's similar or identical to Omega, probably
without the special handling of Omege ^^-codes. One
needs to check what is output by XeTeX, the archive of
the MacOSX-TeX mailing list has information about that
topic, roughly in June or July 2006.