Add new package type to support converting "curl|bash"-style things to other packages.

[jordansissel]

A few years ago, I demoed how to convert a "curl | bash" to an rpm.

Might be time to add this to fpm?

• https://twitter.com/jordansissel/status/1087967949363863552
• https://gist.github.com/jordansissel/0e0fc22344a17eadd1a3b9a1bbb7ed60

I think the rough steps are:

1. Generate a simple docker file
2. docker build ... it
3. docker save the result
4. Cherry-pick the latest layer (it's a tarfile) in the docker save
5. Convert the tar to whatever format.

It's possible other tools might work for this as well, instead of docker, but we can start with docker.