Update CHANGELOG for 0.2.1-nullsafety pre-release.

Match min dart SDK constraint for stable release

Reduce dart SDK constraints for unsound null safety

For sound null safety, use the -nullsafety pre-releases until such a
time that all of the package dependencies have migrated.

Bump version to 0.2.0-nullsafety.

Tag v0.1.3

Run through dartfmt for publishing

Handle unknown WMIs gracefully, bump to 0.1.1

Add NHTSA DB make/model querying, bump version to 0.1.0

Add missing meta package dependency for @required keyword annotation