Improve plugins structure

[pv8]

Remove plugins from the project so they can be installed and loaded separately from noipy - i.e. Flask extensions structure.

Currently, plugins are specified inside dnsupdater.py file:

noipy/noipy/dnsupdater.py

Lines 15 to 24 in 0fbacd9

	AVAILABLE_PLUGINS = {
	'noip': 'NoipDnsUpdater',
	'dyn': 'DynDnsUpdater',
	'duck': 'DuckDnsUpdater',
	'generic': 'GenericDnsUpdater',
	}
	DEFAULT_PLUGIN = 'generic'

Important: A good documentation on how to implement plugins need to be part of this improvement.