Allow commit message author and committer on delete file action

[ekeren]

The commit message for file deletion is hardcoded, we would like to allow the same api as writeFile

The new signature should be: function deleteFile(branch, path, message, options, cb) instead of existing one: function deleteFile(branch, path, cb)

message, option - should be optional

We already have the code that fixes this issue in our code base, if this request is approved we can open a pull request.

Thanks in advanced

[j-rewerts]

Please submit this as a PR.