Created by: kt3k
Hello, gulp team!
This change allows users to use Gulpfile.js as one of the default filenames of gulpfile.
I don't know whether only allowing gulpfile.js as the default is intentional or not, but Gulpfile.js may seem more natural to the people who migrated from grunt and this also follows the general rule of the capitalization of XXX-files like Makefile, Dockerfile or Gemfile.
Thanks!
The configPathFlag property keeps the current behaviour of --gulpfile option of the command line.