parse number opts accordingly if passed as string

[kaizensoze]

Fix issue in which passing in number opts (width, height, cell_size, etc.)
as string leads to unresponsive script.

Resolves #35

[qrohlf]

Duplicate of #35 - the script encountering errors if you pass the wrong type of object in as an argument is expected behavior, though the error handling could be improved to throw helpful errors and exit instead of hanging.