Created Whitelist Plugin c.f. filter_by_upstream (also fixed latter)

[nimpo]

1. Fixed the Filter By Upstream Plugin so that it compares server-name against the defined/default comma separated filter list of server-names. It was trying instead to match IP address of incoming requesting to the same list of server-names and therefore never matching e.g. the example facebook.com.
2. Made a copy of that Filter By Upstream Plugin, called it Whitelist Upstream Hosts Plugin, and changed code so that the filter behaves in the opposite way: default deny with only matching server-name requests allowed via proxy.
3. Updated init.py to allow loading of plugin.
4. Updated Readme to add info on Plugin.

The reason for this code change was that I wanted a proxy to run on the head node (or associated VM) of an HPC cluster such that a job submitted to the queue, when running on worker nodes with no outbound connection, could set $HTTPS_PROXY to allow singularity to contact the Singularity Hub for required containers to download at runtime. I did not want my proxy to be used as a generic outbound proxy except to pass traffic to Singularity hub and known dependencies at time of writing.

Code was patched on remote HPC which is using an older branch installed via pip. This pull request is against the current development branch, pulled gie pip and git and tested in my local Ubuntu 22.04 environment.

Thank you for your work on proxy.py it has helped work round a singular issue :-)