Make it possible to make "plugins" to hook into the zino2 code.

[runborg]

There are multiple places there would be nice to be able to hook into the zino2 codebase.
examples of this are:

• make a plugin to add/remove/edit monitored devices dynamically without using polldevs.cf
• customize selection on witch interfaces on a device should be monitored or not.
• run custom code on event creation/modification/close/deletion
• interaction with planned maintenance
• adding metadata to devices/ports/events

These plugins needs to be executed as non-blocking bits of code to not to interfere with the zino2 core.
I've not been thinking of the best way to do this, but it could be done using external libraries that is loaded by config and installs predefined hooks around the codebase.
There are multiple ways this could be done.. :)

This also could be the way to address issue #380