text: Title: Adding file.peek() 
  Body: Hello!
  
  I noticed that in your README.md there is no mention of your aiofiles supporting [peek](https://docs.python.org/3/library/io.html#io.BufferedReader.peek). However in your code there are some tests for it. 
  
  When attempting to use peek in my code, it did not seem to work as desired. Could you please add this functionality?
  
  Many Thanks