Add silences fetching

[lekspyl]

Hello,

I think it would be nice to fetch data about silences from api/v1/silences endpoint. It will allow to observe not only active alerts but also silence rules (I believe this is also very important thing to monitor as incorrectly defined silence may mute important alerts which should not be covered by silence etc.)

Related issues #6 and #35 will only give ability to show silenced alerts but this feature request aims on giving ability to show list of silence rules instead.