Teach completion.charset to use CharsetMenu.jsm

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

In Firefox 32, upstream dropped charsetTitles.properties.
https://hg.mozilla.org/mozilla-central/rev/15680e55195c
(https://bugzilla.mozilla.org/show_bug.cgi?id=943268)

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