Allow setting CreateFlag when creating files

[peterfajdiga]

When creating files, CreateFlag is hardcoded to 1 (CREATE) here: https://github.com/colinmarc/hdfs/blob/master/file_writer.go#L69
Often you want a different CreateFlag, for instance CREATE|OVERWRITE.
It would be immensely useful to be able set the CreateFlag when calling Create (or perhaps a new method).