Allow .ftppass file path to be configurable

[dynamicdan]

Would be nice if I can put all my credential files and non-repo files in a folder called secret/ rather than having to exclude one by one each new file in some kind of .ignore file. Would also help keep secret files secret form other devs with less permissions.

    // don't keep passwords in source control
    config: 'secret/.servers.json',

(inspired by https://github.com/andrewrjones/grunt-ssh)

Also, would probably make sense to put host and port info into the json file as the host and port generally don't change for a given authentication process.

[inossidabile]

It's a nice feature that I'd really like to see however I don't have time to invest into it. Pull-Requests are very welcome.