Add combined plural and sprintf function

And exposing it to the public api. The function uses the first argument
to get the correct plural translation, the second argument is the
string to translate and all the following arguments are used for
sprintf. The function also replaces __count__ and %d with %s to use in
the sprintf function.
Exposing the translate and plural function to helper

The original t and plural functions are exposed to use directly in eg.
Express routes.

They are exposed as ‘__’ and ‘_p’, and the ’t’ function is kept as is.
The function exposed as ’t’ does not take several arguments, and thus
have problem with a sprintf string with multiple variables. The new
exposed function may have problem with dynamic language control, and
has only been tested with a fixed language for the Node.js application.