Add _merge support for custom plugin

[vuksv-scotia]

I was attempting to make modifications to the apikeys plugin to use for my organization's use case but discovered that apikeys gets additional config passed to the config object through this microgateway-config project.

There should be a mechanism to specify the name of a plugin perhaps in the config.yaml to which to pass this data from Edge to the config object of that plugin.

The parameters in question being:

path_to_proxy
product_to_proxy
product_to_api_resource
public_key

This could be useful for a custom plugin development to have access to the config from Apigee Edge itself.