Based on most frequent use cases, add a set of basic hooks that third party developers would need #615

We could easily add a set of basic hooks, based on common sense and most basic use cases. We already have the most important ones (add menu, add widget, hook on archiving, etc.) but we could add others:

• on page init, page end
• hooks in the archiving module so that archives of any plugin can be modified, or read and modified in a different archive
• etc.

Hard to guess what users will need. We will add any hook necessary to a new feature on demand