Add support for Ruby format strings.
* gettext-tools/src/message.h (enum format_type): Add format_ruby.
* gettext-tools/src/message.c (format_language, format_language_pretty): Add an
entry for format_ruby.
(NFORMATS): Increment.
* gettext-tools/src/format.h (formatstring_ruby): New declaration.
* gettext-tools/src/format.c (formatstring_parsers): Add an entry for
format_ruby.
* gettext-tools/src/format-ruby.c: New file, based on
gettext-tools/src/format-python.c.
* gettext-tools/src/FILES: Mention it.
* gettext-tools/src/Makefile.am (FORMAT_SOURCE): Add format-ruby.c.
* gettext-tools/doc/gettext.texi (PO Files): Mention ruby-format.
(ruby-format): New section.
