Adde support for additional Script parameters to install and update commands

This pull request is an attempt at allowing scripts to have parameters defined at runtime. 
An example would be to use  >composer install --script-tag="env:--env=prod" on production.

Scripts would then be configured in composer.json with "myscript {env}".

PHP Callbacks would have access to the parameters through the Event object:
$event->getScriptParams();

There are some decisions that need review in my opinion:
-- Is it desirable to add the scriptParams to the Event constructor?
-- Would the InstallCommand and UpdateCommand need more explanation?
-- Is the help page correctly altered?
-- Are there tests needed? I could not find a good place to add tests for this...