Feature: Add plugins support

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Idea:
To make RiotKit Do possible to use for example in non-officially supported languages, to allow to add progress bars, slack notifiers and more we can add plugins support.

Plugins support could be introduced pretty easily using some kind of event bus.

Implementation:

• Use event bus
• In Python syntax and in YAML syntax import plugins via imports section. The difference is that the def imports() could take also PluginDeclaration, not only TaskDeclaration

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https://zopeevent.readthedocs.io/en/latest/classhandler.html
https://pypubsub.readthedocs.io/en/latest/usage/usage_basic.html
https://github.com/seanpar203/event-bus

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Implement example plugin: Progress bar

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All events emmited by RKD MUST BE properly documented. Each event needs to be a class object and have its own type.