Allow for user defined gitignore instead of custom "includes"

[samrocketman]

I'd much rather use my own .gitignore rather than the includes defined in this plugin. Credentials should be included because they're encrypted and protected by the secret.key file.

Additionally, the cloudbees folders plugin stores credentials in the folder config.xml; another reason not to track secret.key but not worry about credentials which are encrypted with AES128 secured with a 512-bit passphrase.