Adds ability to configure default config path

[rossta]

The Litestream Ruby config path defaults to "config/litestream.yml". Some applications may wish to use a custom configuration path, say, one that is based on the Rails environment "config/litestream/#{Rails.env}.yml"

Though Litestream::Commands methods allow for passing a --config option for the config path, helper methods like Litestream.verify!, which wraps Litestream::Commands.databases and the Puma plugin, which wraps Litestream::Commands.replicate, do not.

This changeset adds the ability to configure an application-wide default for the litestream config path allows for use of features like Litestream.verify! and the Puma plugin without otherwise having to modify these features to accept the --config option.