build: i18n: cleanup and respond to review

Respond to review comments from @misterdjules:

* Remove the prepare-icu-source.sh mechanism for creating
  an embedded deps/icu-small as per
  7cb4aad7f3c4da2910146d8ff7750cb27ccb664f - it was not
  currently used and thus deadwood.

* Updated README.md to clarify some of the options:
  that 'none' is the default for --with-intl, and that
  --with-icu-source works with 'small-icu'

* clarified the test-intl.js test case

* cleaned up and added documentation in configure and in nodedownload.py
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.