More features.

New features:

• --version=unstable will include all versions
  containing '-' and '+' from being considered valid.
  As per SemVer, these are the separators for unstable
  versions (i.e. "-RC2", "+abcdef" suffixes).
• --review flag can be used to initiate a run of
  nixpkgs-review, which will ensure all
  dependent packages can be built.
• In order to ensure consistent formatting, the --format flag will invoke
  nixpkgs-fmt.

Bugfixes:

• nix-update now works if checkMeta is enabled in ~/.nixpkgs/config