support for the {plural} tag, in the language and the PO backend.

Limitations:

- When extracting PO messages:
-- Only [1, default] plural cases are supported. If other forms are found, the
   extractor exits with an error

- When loading PO messages:
-- fallback between locales is not implemented.
-- Plural-Forms is not respected. "nplurals=2; plural=(n != 1);" is assumed.
non-English plural forms

Soy syntax uses CLDR plural classes, which are fairly incompatible with PO
plural specification. To work around that, soymsg.Bundle specifies the plural
case to use, which avoids (1) converting PO plurals to their CLDR class and (2)
calculating which CLDR plural class should be used for a given (locale, value).

Usage example:
go install github.com/robfig/soy/soymsg/pomsg/xgettext-soy
xgettext-soy testdata/features.soy > features.pot
msginit -i features.pot -o - -l sk > features.po
.. insert translations in features.po ..