Update the running as non-superuser section. This solves the problem that certain metrics didn't produce a value.

This functionality was added in c225a1e1
but not documented.

Improve the documentation for `--disable-default-metrics` and update the listing of current flags.

This commit implements a massive refactor of the repository, and
moves the build system over to use Mage (magefile.org) which should
allow seamless building across multiple platforms.

* In docker example both containers should run on the host net 
* remove unecessary portmapping in docs

Revamp the build system to be more inline with other Prometheus exporters.
Notably add Darwin and Windows build targets, and add support for releases
using tar files.

In PostgreSQL, views run as the defining user, meaning you don't 
need a view wrapping a function (you can just have a view).  This 
improves readability, maintainability, performance, and clarity.

If the deployer role is not superuser (which is the case in AWS RDS,
for example), we need to grant it access to postgres_exporter role
before we can create a schema with AUTHORIZATION postgres_exporter.

PostgreSQL name was misspelled in
53b9d9ce.

Closes #53 (rebased and merged).

While many pronounce it as 'post-GRES', the spelling is 'PostgreSQL' with two 's'.

https://www.postgresql.org/about/press/faq/

This was something that was going to be added but never materialized into a practical feature.

Looks like lib/pq has updated their connection strings upstream so the URL parsing is more URL-like.

Vendor and store all dependencies. This allows us to have a makefile which
builds to a minimized docker image.

Added metrics for locks per database per mode, pg_stat_activity, pg_stat_replication.
Updated README.md with possible solution for running as non-superuser.

Update README with a workable connection string for local use.

This is based on the mysqld_exporter, but drastically changes how metrics are
discovered and enumerated since postgres already very closely follows the
Prometheus format.