Add configuration to ignore files when running `git status` checks

[mrdavidburns]

The addition of git status check as part of the Static Tests is a nice addition.
However, I can see some projects where there could be a rare exception for a file to not be committed to git.

Could we add configuration to drainpipe's composer.json for this functionality?

proposed solution:

"extra": {  
      "drainpipe": {
            "github": [
                "PantheonReviewApps",
                "Security"
            ],
           "git-assume-unchaged": [
                ".tugboat/config.yml",
            ],
        },
    },
}

[deviantintegral]

I don't see how the above example could work. Tugboat reads config.yml when it builds a preview, so any changes made to the file won't take effect?