Allow for optional external viewer args

In order to pass in options like `--new-window` to an external viewer like
`firefox`, optional args can now be defined in configuration. See
config-example.yaml for an example.

cui/colours.go

@@ -0,0 +1,8 @@
+package cui
+
+import "github.com/jroimartin/gocui"
+
+const (
+	selectionBgColor = gocui.ColorWhite
+	selectionFgColor = gocui.ColorBlack
+)