RFE: multiple extra src args

[vbatts]

allow providing multiple extra src directories for layering content, rather than putting many sources in a single directory.

This will be either via making ${input_extra_src_dir} a bash array, and appending to it via getopts
OR
Doing something nasty like comma or colon delimiting the ${OPTARG} already there, so a single flag does the trick for handling multiple directories.