Allow custom placeholder image and use transparent_image for default

[kaetemi]

This fixes continuous exceptions caused by the previous default MemoryImage being initialized with new Uint8List(0)

I/flutter (10904): Another exception was thrown: Exception: operation failed
E/flutter (10904): [ERROR:flutter/lib/ui/painting/codec.cc(83)] InitCodec failed - buffer was empty
I/chatty  (10904): uid=10093(net.nbspou.infapp) 1.io identical 17 lines
E/flutter (10904): [ERROR:flutter/lib/ui/painting/codec.cc(83)] InitCodec failed - buffer was empty
I/flutter (10904): Another exception was thrown: Exception: operation failed

[kaetemi]

Fixes #22

[johnpryan]

groovy, thanks!

[kaetemi]

Thanks for the great mapping widget too :)