[suggestion] Allow `Gulpfile.js` as one of the defaults

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!

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The `configPathFlag` property keeps the current behaviour of `--gulpfile` option of the command line.