Enforce `prefer_is_empty`

Merge pull request #9 from davidmorgan/enable-prefer-is-empty

Enforce prefer_is_empty.

Update lints

• Enforce unawaited_futures. Stop enforcing control_flow_in_finally and
  throw_in_finally.

Make `analysis_options.yaml` available for import

• Moved analysis_options.yaml under lib so you can import it directly from
  your own analysis_options.yaml. See README.md for example.

Initial release with `unawaited`

• Describe Dart static analysis use at Google in README.md.
• Add sample analysis_options.yaml.
• Add unawaited method for silencing the unawaited_futures lint.