Add flags support (fix #49)

[raphink]

Use github.com/jessevdk/go-flags instead of github.com/caarlos0/env

$ Usage:
  conplicity [OPTIONS]

Application Options:
  -i, --image=              The duplicity docker image. (default: camptocamp/duplicity:latest) [$DUPLICITY_DOCKER_IMAGE]
  -u, --url=                The duplicity target URL to push to. [$DUPLICITY_TARGET_URL]
  -g, --gateway-url=        The prometheus push gateway URL to use. [$PUSHGATEWAY_URL]
      --aws-access-key-id=  The AWS access key ID. [$AWS_ACCESS_KEY_ID]
      --aws-secret-key-id=  The AWS secret access key. [$AWS_SECRET_ACCESS_KEY]
      --swift-username=     The Swift user name. [$SWIFT_USERNAME]
      --swift-password=     The Swift password. [$SWIFT_PASSWORD]
      --swift-auth_url=     The Swift auth URL. [$SWIFT_AUTHURL]
      --swift-tenant-name=  The Swift tenant name. [$SWIFT_TENANTNAME]
      --swift-region-name=  The Swift region name. [$SWIFT_REGIONNAME]
      --full-if-older-than= The number of days after which a full backup must be performed. (default: 15D) [$CONPLICITY_FULL_IF_OLDER_THAN]
      --remove-older-than=  The number days after which backups must be removed. (default: 30D) [$CONPLICITY_REMOVE_OLDER_THAN]
  -b, --blacklist=          Volumes to blacklist in backups. [$CONPLICITY_VOLUMES_BLACKLIST]

Help Options:
  -h, --help                Show this help message

exit status 1

[mcanevet]

Update README?

[mcanevet]

LGTM