PR proposal: StageEnvironment behaviour

[logemann]

Hi,

i needed different behaviour of this plugin when requests are made to a staging environment for several reasons like visual test automation (where you dont want that cookie banner to appear) and other reasons (like local development).

If you like, i could create a PR for my changes made at https://github.com/logemann/gatsby-plugin-cookiehub-banner/
Most notable changes are described in the README (https://github.com/logemann/gatsby-plugin-cookiehub-banner/blob/master/README.md). See stageMode and stageHosts Parameter.

This code is not yet battle tested. Just finished it.