Add CLI support for slack config

[matthew-cox]

Additionally, move the script comments into POD. Try perldoc ./nagios.pl

Allow CLI usage help:

    $ ./nagios.pl -h
    Unknown option: h
    Usage:
        nagios.pl [--help] [-field slack_SOMETHING=value]

      -h, --help:
        This output

      -field:
        Provide slack related configuration. Possible choices include:

          * slack_channel - Target Slack channel for the message
          * slack_domain  - Slack domain for your team
          * slack_token   - Slack token for this integration

        NOTE: Providing the Slack token on the CLI might allow it to be read
        from /proc or the like.

Add nagios.pl specific README file (generated from POD).