Add "execute" service

[exetico]

This service allow users to execute scripts, without having to call a "class_set" or simular, to clean things up - and prevent the usage of ie "none.none" service.

Usage is simple as:

hold_action:
  - action: call-service
    service: floorplan.execute
    service_data:
      bob: |
        > console.log("Here we go again...");
        dwojwo
        functions.testFunction2("Here we go again", entity, entities, hass, browser_mod);
      coffee: |
        > 
        try{
          console.log("this",this);
          console.log("entitiy",entity);
          console.log("elements", elements);
        }catch(e){console.log("Error", e)}

The service will execute every key within service_data, or the string provided to service_data (without any keys).

handleError is changed a bit. Now the error-message is prioritized before the error-stack. A message about "check the browser console" is added, for people without the knowledge about that.

Check this discussion, if you'd like to see my previous toughs about this:
#181 (reply in thread)

And this, if you're curious on why I changed my mind:
#252

[exetico]

Note: Re-added property, removed by mistake (eslint asked for it)