Add ability to generate multiple placeholder images about bootpack HOT 2 CLOSED

Another (simpler) approach would be to modify the current npm run generate:images... command to append the file width & height dimensions + a _### value to the output image name.

Example:

• Run npm run generate:images width 200 height 200; creates: placeholder-200x200_001.jpg
• Run the exact same command again; creates: placeholder-200x200_002.jpg
• Run npm run generate:images width 300 height 300; creates: placeholder-300x300_001.jpg

Either approach would be acceptable. Either approach will likely need to append and increment a _### value onto the filename in order to avoid conflict with multiple files of the same name.

from bootpack.

Addressed in #168

from bootpack.