Modify cpplint.py to allow CPPLINT.cfg overrides

Added the ability to provide CPPLINT.cfg files to provide
linter message filters per sub-directory and special exclusion
rules. Each file can have instructions like:

filter=-build/include_order,+build/include_alpha
exclude_files=.*\.cc

The above disables build/include_order warning and enables
build/include_alpha as well as excludes all .cc from being
processed by linter, in the current directory (where the .cfg
file is located) and all sub-directories.

Patch by avakulenko@google.com.
Related CL: https://codereview.chromium.org/406373002/

Review URL: https://codereview.appspot.com/115340043

git-svn-id: http://google-styleguide.googlecode.com/svn@135 955884f1-7149-0410-9467-59e7ac3f4d80