text: Title: Add recursive option to `repository.tree(sha, recursive=False)`
  Body: # Overview
  
  GitHub Tree API allow to get tree recursively - https://developer.github.com/v3/git/trees/#get-a-tree-recursively
  
  # Ideas
  
  It should be pretty simple for now it works even like this (a hack):
  ```python
  repository.tree('sha?recursive=1')
  ```
  
  
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