Add a `--age` flag (or equivelent) to clear cache command

[joshmanders]

Should probably also add the image creation time to the cached image filename so that way we can add a flag to the command that will allow you to clear out any cached images older than that age. This will allow you to set a schedule command to run say nightly to clean up any images generated older than 30 days ago.

This will keep from filling disks with images that aren't needed