Add script to generate "library/postgres" for stackbrew

Example output:
```console
$ ./generate-stackbrew-library.sh
# maintainer: InfoSiftr <github@infosiftr.com> (@infosiftr)

8: git://github.com/docker-library/postgres@8e917a6ba18d0b8c654c9f699472829fdcd554f9 8.4
8.4: git://github.com/docker-library/postgres@8e917a6ba18d0b8c654c9f699472829fdcd554f9 8.4
8.4.22: git://github.com/docker-library/postgres@8e917a6ba18d0b8c654c9f699472829fdcd554f9 8.4

9.0: git://github.com/docker-library/postgres@8e917a6ba18d0b8c654c9f699472829fdcd554f9 9.0
9.0.18: git://github.com/docker-library/postgres@8e917a6ba18d0b8c654c9f699472829fdcd554f9 9.0

9.1: git://github.com/docker-library/postgres@8e917a6ba18d0b8c654c9f699472829fdcd554f9 9.1
9.1.14: git://github.com/docker-library/postgres@8e917a6ba18d0b8c654c9f699472829fdcd554f9 9.1

9.2: git://github.com/docker-library/postgres@8e917a6ba18d0b8c654c9f699472829fdcd554f9 9.2
9.2.9: git://github.com/docker-library/postgres@8e917a6ba18d0b8c654c9f699472829fdcd554f9 9.2

latest: git://github.com/docker-library/postgres@8e917a6ba18d0b8c654c9f699472829fdcd554f9 9.3
9: git://github.com/docker-library/postgres@8e917a6ba18d0b8c654c9f699472829fdcd554f9 9.3
9.3: git://github.com/docker-library/postgres@8e917a6ba18d0b8c654c9f699472829fdcd554f9 9.3
9.3.5: git://github.com/docker-library/postgres@8e917a6ba18d0b8c654c9f699472829fdcd554f9 9.3

9.4: git://github.com/docker-library/postgres@8e917a6ba18d0b8c654c9f699472829fdcd554f9 9.4
9.4-beta2: git://github.com/docker-library/postgres@8e917a6ba18d0b8c654c9f699472829fdcd554f9 9.4
```