handle custom importer

[stefanocudini]

hi,

for my case study I was need to write a custom importer for specific opendata of my region: Trentino Alto Adige in Italy.
handle in the docker compose here:
https://github.com/DigitalCommonsLab/docker-pelias-geocoder/blob/master/projects/italy-trentino/docker-compose.yml#L130
this importer is published here:
https://github.com/DigitalCommonsLab/pelias-importer-trentino-opendata

I'm created this simple project based on Portland metro:
https://github.com/DigitalCommonsLab/docker-pelias-geocoder/tree/master/projects/italy-trentino
and unfortunately to use this I had to make a change to the base scripts about download,prepare phases here:
https://github.com/DigitalCommonsLab/docker-pelias-geocoder/blob/master/cmd/download.sh#L12

now I wanted to send you a PR to share the work done in italy-trentino.
But I realize that the changes inside the /cmd directory to the base scripts are not compatible with this project

I wanted to ask you if you have foreseen a way to integrate custom importers like my case.
I think for example to transform the script /cmd/download.sh into a directory /cmd/download.d/*.sh directory and run the scripts for all importers(*nix style)

or refact the filesystem of this project differently to be extensible