Support multiple values in the key

[hkcomori]

Closes #64

This PR is an addition to the feature #64.

It allows multiple lines of the same key, different values, to be defined in unit files.

unit_config:
  - name: "test-service"
    type: service
    Unit:
      Description: This is test service
    Service:
      Type: simple
      Environment:
        - "ENV1=foo"
        - "ENV2=bar"
        - "ENV3=baz"
      ExecStartPre:
        - "/path/to/pre1"
        - "/path/to/pre2"
      ExecStart: "/path/to/start"
    Install:
      WantedBy: multi-user.target