Webhook notifications

[Patrik-Stas]

App idea:

• listen on indyscan-daemon websockets
• send slack notification / generic webhook call if pre-configured parameters are met
• there would have to be some sort of UI which would enable users to configure their notifications criteria
• assuming we would properly expose indyscan-daemon websockets, this service could be self-hosted by its users (eg listen to wss://indyscan.io/SOVRIN_MAINNET from self hosted server running this notification app), so no need to implement authentication in the first iteration

Marking as good first issue, as this can be done pretty much without knowledge of the existing codebase. I am sure this would be handy feature for number of people/companies.