Allow the use of `i18n:singular` and `i18n:plural` as directives and not just as attributes. If `py:strip` is involved on `i18n:singular` or `i18n:plural` directives, only apply the strip directive once the singular or plural form has been chosen and translated. Fixes #371.

<div id="header">
  <h1><?cs var:title ?></h1>
</div>

<?cs def:greeting(name) ?>
  <p>Hello, <?cs var:name ?>!</p>
<?cs /def ?>