Allows language packs to specify an array of strings (one for each
plural form in the language) instead of a single string.

The first of the variable (sprintf) arguments is assumed to be the
number we need to check to choose a plural form.  This argument is
passed into a language-specific function defined in langconfig.php,
which returns the appropriate plural form based on a formula.

A similar scheme is used for strings formatted by the javascript
get_string function in mahara.js.

See http://www.gnu.org/s/hello/manual/gettext/Plural-forms.html



Change-Id: Ifb58ac4f1e13a54edbc57e5c0a9faaf8454e53a4
Signed-off-by: Richard Mansfield <richard.mansfield@catalyst.net.nz>