Composer commands (require, install, dump-autoload)

Also written: composer require, composer install, composer dump-autoload

`composer require` downloads a package such as `league/csv` into `vendor/`, adds it to `composer.json` and records the version in `composer.lock`. `composer install` downloads exactly the versions in `composer.lock`, and `composer dump-autoload` rebuilds the autoloader from `composer.json`.