Support for pip 10

[tylerphelan]

Pip 10 no longer supports the use of internal APIs: https://groups.google.com/forum/#!topic/pypa-dev/JVTfS6ZdAuM.

This causes pip-grep and pip-diff to error with:

File "pip-grep", line 12, in <module>
       from pip.req import parse_requirements
   ImportError: No module named 'pip.req'

my pull request #18 tend to repair it. Could you review and comment it, please?