build: i18n: Don't auto-download ICU unless --download=all

Instead of downloading ICU if missing from `deps/icu`, just print a warning
unless either `--download=all` or `--download=icu` is set.

Add some generic scaffolding in case other modules end up in the same boat
in the future.

It's harmless to pass `--download=xyzzy`, so, future proof
(at least until some xyzzy dependency is integrated).
Merge pull request #21 from srl295/srl-v0.12-downloadopt

build: i18n: Don't auto-download ICU unless --download=all

Thanks @trevnorris for taking a look.