Add "addSymbols" and "addSymbolsSize"

[tomroh]

• [x] decide on 1 or 2 functions
• [x] limit to first 7 shapes
• [x] use addMarkers
• [x] how to pass args
• [x] add test

[tomroh]

499159fe0097a22b898aed0d8cb662eff833c3b8