Use triggered_by option for extensions argument for phpcs. Fixes #715.

[hussainweb]

Q	A
Branch	master for features and deprecations
Bug fix?	yes (missing phpcs functionality)
New feature?	yes
BC breaks?	no
Deprecations?	no
Documented?	yes
Fixed tickets	#715

I have given a detailed description in #715.

New Task Checklist:

• Is the README.md file updated?
• Are the dependencies added to the composer.json suggestions?
• Is the doc/tasks.md file updated?
• Are the task parameters documented?
• Is the task registered in the tasks.yml file?
• Does the task contains phpspec tests?
• Is the configuration having logical allowed types?
• Does the task run in the correct context?
• Is the run() method readable?
• Is the run() method using the configuration correctly?
• Are all CI services returning green?

[hussainweb]

CI is green. Several of those tasks are not relevant here given the nature and size of the problem. I have ticked them as well. I hope that's alright.

[veewee]

Thanks for the PR! Looks good.