Feature Request: Save/load Ginga shape as non-FITS image

[pllim]

As requested by @nluetzge:

Currently, Ginga's Drawings plugin can save out drawings as FITS image (2D array that can be used as a mask). However, it would be advantageous storage-wise to also let user save the drawings out in a non-image format (e.g., #590 but other non-DS9 formats are also acceptable). Ginga then should also be able to read this format back in.