ci: add tool to check for Cargo features

When running tests from .travis.yml, the passed in features are first
checked locally for support in the current simulator. The list of
supported features was manually maintained, allowing newly implemented
features to be skipped, also skipping the related test (without
warnings). This adds a new tool that parses and prints the list of
features directly from the given Cargo.toml.

Signed-off-by: Fabio Utzig <utzig@apache.org>