Plugins for composer

[dsvoronin]

We in Avito customized composer heavily (not oss). Main reasons:

1. we need separate instrumentation and dynamic sharding (will be addressed in #135)
2. also customization of junit report based on custom annotations on test methods/classes

Regardless .2 dex-test-parser(will be used in #135 ) already provides ability to read annotation values, but same issue contains valid concerns: "if this is a right place to have custom logic, or maybe better have test side-effects(write meta to file and parse it afterwards as artifacts)".

If composer is the right tool for this customization we need some sort of plugin mechanism.
How this can be done?