New composer dependency: globalis/wp-cubi-helpers

[pierre-dargham]

New dependency on https://github.com/wp-globalis-tools/wp-cubi-helpers/

Before we merge this, we should decide:

• How do we generate wp-cubi-helpers markdown documentation from DocBlock comments ?
• Where do we put this documentation ?
• How should wp-cubi README.md refer to this documentation ?