Ability to read header from separate reader has been added

[OptimumCode]

I added the ability to append data to an existing avro file when you can't use it as the os.File (read and write using one structure). It's applicable for HDFS, for example, when you can read data from the file beginning and append data to the end of a file only.

I didn't add reading all data from the passed Reader because the existing file can we large and reading can take much time. So, we can't check the file structure before writing because of my decision. But if you prefer I could add an additional parameter to define that data from the passed Reader should be read to the end or not.