Add alias to require command

[dmitryuk]

I think require command is one of most frequency used by developers. Why do not create shortcut alias for it?
Most commands like install or update already have shortcut aliases.
Much easy to write composer r package:version instead of composer require package:version. It will also pretend typo in require word within fast-writing cases.

The PR kindly adds r alias to require command.

[Seldaek]

Just so you know, you can already use composer req which is non-ambiguous with remove and reinstall. re conflicts woth those two and r as well, so that doesn't work. But yes I agree we can add an alias for require that sounds sensible.