Option to exclude files from ament_auto_lint about ament_lint

[jacobperron]

cc/ @AAlon

[emersonknapp]

I believe this was closed with #386

[methylDragon]

This issue was also duplicated in #343

I think I will close this.

---

To exclude files, simply:

    # this must happen before the invocation of ament_package()
    if(BUILD_TESTING)
      find_package(ament_lint_auto REQUIRED)
      set(AMENT_LINT_AUTO_FILE_EXCLUDE /path/to/ignored_file ...)
      ament_lint_auto_find_test_dependencies()
    endif()