Drawing on the map

It would be great to have an API to draw on the map. For starters I can envision three shapes:
- [ ] **Lines**. A constructor could look like:

``` js
new Line({
points: [ [lat, lon], [lat, lon], ... ],
strokeColor: "blue",
strokeWeight: 2
});
```

Similar to the other Tabris.js APIs the various color paramaters could take a [`Color`](https://tabrisjs.com/documentation/latest/types#color) which would eliminate the need for a separate key to specify opacity
- [ ] **Polygons**. Similar to Line but with optional stroke color and fill. There would probably be an implied connection between the first and last points if they differ. A constructor could look like:

``` js
new Polygon({
points: [ [lat, lon], [lat, lon] ],
strokeColor: "blue",
strokeWeight: 2,
fillColor: rgba(50, 50, 255, 0.5)
});
```
- [ ] **Circles**. A centerpoint and a radius A constructor could look like:

``` js
new Circle({
center: [lat, lon],
strokeColor: "blue",
strokeWeight: 2,
fillColor: rgba(50, 50, 255, 0.5)
});
```