Add extension mechanisms for custom checks to cpplint.py.
Add extension mechanisms for custom checks to cpplint.py.

This allows users of cpplint.py to specify additional check functions.

Will be used by: http://codereview.chromium.org/7834045/

Review URL: http://codereview.appspot.com/4950069
Patch from Alexei Svitkine <asvitkine@chromium.org>.
google/styleguide,theandrewdavis/cpplint,google/styleguide,google/styleguide,google/styleguide