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.

Related CL: https://chromium-review.googlesource.com/#/c/209384/

BUG=chromium:395296
TEST=ran cpplint.py on a bunch of directories in platform2/

Review URL: https://codereview.chromium.org/406373002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@285999 0039d316-1c4b-4281-b951-d872f2087c98