add --batch parameter

[SivanMehta]

Allows passing the --batch parameter on the command line so that more than one image can be generated at once. By default, this is set to 1 to replicate existing functionality.

When using batched generation, output is updated to indicate each file that is generated:

python text2image.py --prompt="nothing" --output="test.png" --batch=4

# generator output

saved at test-1.png
saved at test-2.png
saved at test-3.png
saved at test-4.png

[SivanMehta]

Copied here: divamgupta#29