Make all messages i18n friendly

[mattupstate]

Voluptuous is awesome, but there are some baked in messages that I'd love to be able to translate with babel (or whatever other tools you might use). Not sure how one might approach this, but perhaps we can brainstorm.

[alecthomas]

I'd be amenable to this, but have Zero experience with i18n. I'd definitely welcome patches if they're well thought out.

[aleksandr-kotlyar]

I opened #547 with a proposed implementation for this.

The PR adds gettext-compatible runtime hooks for Voluptuous validation messages, without shipping production translation catalogs. Applications can provide their own catalogs via configure_i18n(), inject a custom callable with set_gettext(), or use gettext_scope() for temporary context-local overrides.

I also kept the @ data[...] path suffix untranslated so existing code that parses validation paths should remain stable.