BREAKING CHANGES:

This release changes support for multiple postgres servers to use the
multi-target exporter pattern. This makes it much easier to monitor multiple
PostgreSQL servers from a single exporter by passing the target via URL
params. See the Multi-Target Support section of the README.

* [CHANGE] Add multi-target support #618
* [BUGFIX] Add dsn type for handling datasources #678

Signed-off-by: SuperQ <superq@gmail.com>