Teach completion.charset to use CharsetMenu.jsm

In Firefox 32, upstream introduced CharsetMenu.jsm and dropped
charsetTitles.properties.
https://bugzilla.mozilla.org/show_bug.cgi?id=943268

This teaches `:set encoding=…` to work again on Firefox >= 32.

Signed-off-by: Takeshi Banse <takebi@laafc.net>