Added Magento2 CLI completions (#4043)

* Added Magento2 CLI completions

This is the completion file for the Magento2 CLI application I use on my servers. It has an additional feature tho, I'm not sure if it fits into the fish completion philosophy:
If you provide limited access credentials, it will connect to the MySQL database and provide additional suggestions, such as available users, themes or indexers in the database. If this file is never touched, those suggestions simply won't show up. I, personally, find them to be pretty useful, though.

Should I remove those database suggestions before creating a PR?

* Removed functions using MySQL, updated formatting

* Several smaller fixes

* Improved descriptions

Tried to shorten the text as much as possible and removed unnecessary characters

(cherry picked from commit 71f5fe1)