Add an option for setting a default zoom level in visualization component

[ionosphere]

when we try to show a point , the default zoom is too hight.

i wish i could write something like this

= visualization do |v|
          - v.background "Esri.WorldImagery"
          - v.serie :data, [{name: resource.number, shape: resource.geolocation}]
          - v.path :name, :data
          - v.control :zoom, default_value: 15
          - v.control :scale
          - v.control :fullscreen
          - v.control :layer_selector

[PH64]

With the new settings backgrounds V2.0 RC17, this option is added