Added new iOS mutedStandard map-type

[IjzerenHein]

Hi, this PR adds support for the new mutedStandard mapType for iOS 11. This mode makes the map-tiles/streets/etc.. a bit more subtle, so that the markers and polylines to pop-out more.

mapType="mutedStandard"

mapType="standard"

Recommended use:

<MapView
  mapType={((Platform.OS === 'ios') && (parseInt(Platform.Version, 10) >= 11)) ? 'mutedStandard' : 'standard'}
 ...
 >
  ...
</MapView>