0.12.0: Add test demonstrating the use of alt templates dir (#128)

Changes:
- Added `files` attribute to `helm_package` for supporting the [`.Files`
api](https://helm.sh/docs/chart_template_guide/accessing_files/)
- Added `template_patterns` to `helm_template_test` for adding
assertions that certain values are rendered into from the templates in a
helm chart
- Fixed infinite loop when templates are placed in a directory not named
`templates`

(closes https://github.com/abrisco/rules_helm/issues/106,
https://github.com/abrisco/rules_helm/pull/102,
https://github.com/abrisco/rules_helm/pull/123)