Allow gulp task list to be extended.

As a starter kit, we want users to be able to customize.
As a source-based distribution we want users to stay updated.
By allowing users to customize web-starter-kit gulp
without editing gulpfile.js, they can rebase or merge future updates.

Impl: Apply gulp recipe 'Split tasks across multiple files'
https://github.com/gulpjs/gulp/blob/master/docs/recipes/
split-tasks-across-multiple-files.md