create (language) extensions on dbtest, dbfakeinstall dbinstall

We were trying to create plperl procedural language with a patch, but this was disappearing from the base.sql since the languages are not associated with the main schema.  Instead, do a "CREATE EXTENSION IF NOT EXISTS $language" each time you do a dbtest, dbfakeinstall or dbinstall  (alex prefers create extension to create language, because the language shows up when you do a \dx).  Wrote an example, a test and added some basic documentation for the feature.