* Add exclude-databases option
* Update readme to explain --exclude-databases
* Add comments to ExcludeDatabases function and unexport Contains function
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.
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.
