Add the postgresql_extensible plugin

This plugin is intended to add an extended support of Postgresql
compared to the legacy postgres plugin.

Basically, the plugin don’t have any metrics defined and it’s up to the
user to define it in the telegraph.conf (as a toml structure).

Each query can have it’s specific tags, and can be written specifically
using a where clause in order to eventually filter per database name.

To be more generic, a minimum postgresql version  has been defined per
query in case you have 2 different version of Postgresql running on the
same host.