[Go version] Support file-based value files

[lucasreed]

Is your feature request related to a problem? Please describe.
Should support parsing files (most likely being done in the python version by passing --values <file_path/file_name to the helm command utilized) as values files like so in the course.yml file:

charts:
  chart-one:
    repository: fairwinds-incubator
    chart: basic-demo
    files:
      - subfolder/yaml_values.yml