Add support for --extension in pg_dump

Add support for --extension in pg_dump

When specified, only extensions matching the given pattern are included
in dumps.  Similarly to --table and --schema, when --strict-names is
used,  a perfect match is required.  Also, like the two other options,
this new option offers no guarantee that dependent objects have been
dumped, so a restore may fail on a clean database.

Tests are added in test_pg_dump/, checking after a set of positive and
negative cases, with or without an extension's contents added to the
dump generated.

Author: Guillaume Lelarge
Reviewed-by: David Fetter, Tom Lane, Michael Paquier, Asif Rehman,
Julien Rouhaud
Discussion: https://postgr.es/m/CAECtzeXOt4cnMU5+XMZzxBPJ_wu76pNy6HZKPRBL-j7yj1E4+g@mail.gmail.com