lowercase-dash rule allows for underscores

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The lowercase dash rule doesn't catch file names that have an underscore.

For example ./bin/oath2_proxy won't trigger an error with the following rule:

{
  "rules": [
    {
      "files": [
        "bin/**/*"
      ],
      "naming": "lowercase-dash",
      "level": "error"
    }
  ]
}

I know the rule is working because if I rename the file to ./bin/oath2_proxY fs-lint will trigger the error:

error: violates 'lowercase-dash' naming: bin/oauth2-proxy-2-2-0-linux-amd64-go1-8-1/oauth2_proxY