Supports injecting info file metatdata into YAML files.

- Fixes issue #943
Core check should be future-proof, not 8.x-specific.
Use heredoc templates for info metadata.
Metadata rewriting matches drupal.org.

- Uses double quotes for .info files
- Uses single quotes for .info.yml files
- Adds a yaml metadata rewrite test
Merge pull request #944 from drush-ops/yaml-info-file

Supports injecting info file metatdata into YAML files.