In lint-webkitpy: add help message + non-behavioral change to run_pylint.

Specifically:
 - Refactor _run_pylint
 - Rename it to be run_pylint since it's used outside the class
 - If no arguments are given to lint-webkitpy, print a usage message.

Review-Url: https://codereview.chromium.org/2080053002
Cr-Commit-Position: refs/heads/master@{#401336}
2 files changed