Implement Composer plugin

[2martens]

This task is about implementing the Composer plugin.

The goal of the Composer plugin is to hook into the post-package-install script event, extract the composer ID of the installed package (e.g. 2martens/composer-plugin) and create a file in the plugin directory (cooperate with the AppKernel issue to find out where it will be). The filename will be composed of an integer, which is received from the index file in said directory, an underscore and then the composer ID (slash replaced with underscore). An example for the filename: 0_2martens_composer-plugin. The file contains the PHP code necessary to instantiate an object of the Bundle class of the package (WIP).

This task contains a list of TODOs, each representing a logical sub exercise that can be tackled one at a time. They stand in the chronological order in which they should be worked on. In parentheses you can find the estimated net work amount required. This is a rough estimate and can be quite a bit off (in either direction). When you work on the issue, please update the text with the actual net work time you needed. This can then be used for improvements in upcoming estimations.

• define public API of class (estimated 1 hour amount of work)
• write test class (estimated 1.5 to 2 hours amount of work)
• implement class including test iterations (estimated 1 to 1.5 hours amount of work)
• write documentation in docs repository (estimated 1 hour amount of work)