message: add base for opencv major version selection
diff: @@ -32,8 +32,10 @@ rayon = "1.0"
  matches = "0.1"
  [features]
  -docs-only = []
  +default = ["opencv_34"]
  buildtime_bindgen = []
  +opencv_34 = []
  +opencv_41 = [] # not useful yet
  [package.metadata.docs.rs]
  features = [ "docs-only" ]
  