Add postgis installation

[gma2th]

This is a draft of what could be the installation of extensions with pgenv. Tested with PostgreSQL 12.0 and Postgis 3.0.0beta1

Exemple:

$ pgenv use 12.0
$ pgenv extensions build postgis-3.0.0beta1
$ psql -c "create extension postgis"

Limitation:
Only postgis supported
No format of available extensions
No cleaning

#33

[theory]

I wonder if we should add a directory for shell scripts that install specific extensions or something. Might be able to generalize a lot of them by installing from PGXN (API docs and client). PostGIS isn't on PGXN, but a lot of other extensions are.

[fluca1978]

I think we could add some sort of "post-install" configuration variable with the path of a program to run. And more in general we could add "hook scripts" to run before and after every build/install step.

[fluca1978]

I don't like the proposed implementation too much.
I think we should rely on external scripts, e.g., pgxnclient.
Since I'm looking at a (very simple) implementation of running psql with the right version, making an external shell script that relies on pgenv psql should make it even simpler ( see #38 ).
I don't think we need some as complex as patching, but since it is already there the whole directory/versioning structure, we could work on that and place a script in the same tree to be run once use is launched for the first time (i.e., initdb runs), or when make ends.

[fluca1978]

@gma2th can you try installing postgis as a POSTINITDB script out of the current
b40b942 ? If that results general enough we could also include it as an example to activate on demand.