Add Fluent language functions

[alerque]

I have a need for a CLI tool tool that not only implements data templating but also gives access to localizations through Fluent. There are several tools for the Handlebars ecosystem (at least JS and Rust ones that I'm aware of) that make this possible but I would prefer to use Tera if possible. There doesn't seem to be anything out there yet, but the fluent_templates crate already has support for use as a Tera function, so it shouldn't be hard to bring the existing library functionality to a CLI.

Is this project interested in adding this functionality (either as a default feature or behind a feature flag), or should I consider forking or creating an all new project for this?

c.f. chevdor/tera-cli#20