Support for constants in JSON stylesheets and Swift 5.0

• You can now define and reuse constant values inside a JSON stylesheet, thanks to @lucaslt89

  Example:

  { 
     "constants" : {
        "PrimaryBackground": "#3849DE"
      } ,
      "styles" : [
         {
            "styleName": "DefaultButton",
            "styleProperties": {
               "titleColorForNormalState": "#D35400",
               "backgroundColor": "$PrimaryBackground",
               "borderColor": "#FDE3A7",
               "cornerRadius": 6.0
        }
     ] 
  }
  

• The Swift version is now bumped to 5.0, also thanks to @lucaslt89