Fix black bounding box issue

Fixes an issue where a too-small canvas or too-large word list would
cause WordCount to crop outward, resulting in black bars in the image.
Adds allow_resize option which, when enabled, will correctly resize the
canvas to a larger dimension.
Fix out-of-sync get_image_map() results

FrequencyTable allowed empty strings to be inserted into the table,
which caused the image map to be out of whack.