Move plugins to their own module

[sknat]

Move plugins to own module

This patch moves the binapigenerator plugins to their own package.

• It capitalizes the methods they leverage.
• It changes the way plugins are called, passing the whole generator at once and letting the plugin themselves loop on the files. This allow for more variety in filtering, aggregation, etc...

[ondrej-fabry]

Closing as this will be implemented by #93.