Consider enabling Fluent language functions by default

[alerque]

From #20:

I would kindly request you reconsider whether [the Fluent feature] should be enabled by default. My opinion and reasoning go like this: the people who care about the compile time and binary size (which are admittedly both heftier with the expanded dependency chain) are the ones most likely to have the tooling at hand to run cargo build --no-default-features themselves and get exactly what they want. On the other hand the people who [get their apps] by some other means such as prebuilt binaries, or more notably via distro package managers, are the least likely to be able to easily spin up their own tooling to get the full feature set added in [and the least likely to care how long it took to build]. I'm the one packaging for Arch Linux (and will likely promote this to the [extra] repository soon) and will be adding the feature flag by default, but what about people that install via Homebrew or apt or whatever? Those people will not care about the build time at all since it doesn't affect them and are unlikely to care much about the binary size either. On the other hand they might care that their app doesn't have the full feature set.

I think having features off by default makes a lot more sense for Rust libraries where the downstream compile time might be a significant consideration and the users (i.e. projects that spec dependencies) can much more easily add the features they need. I don't think it makes much sense to have them off by default for CLI tooling that people are likely to get in binary form from channels that don't provide options.