Add CLI flag to verify config and exit

[zswanson]

This change adds a CLI flag to verify the config file/options load successfully, then exit. This allows verification of config files in CICD, where the app needs to gracefully exit or can't easily run in a managed way.

Output on valid config:

 $ ./prometheus-nginxlog-exporter -config-file example-config.hcl -verify-config true
loading configuration file example-config.hcl
Configuration is valid

[martin-helmich]

Thanks for your contribution! 👍 in general on this change -- validating config files in a CI/CD context was not something I've had on my radar. Given just a tiny bit of documentation, I'll be happy to merge it as-is.

[codeclimate]

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