Store current project-level upstream setting and reset when unloaded

[jpasski]

The current project-level upstream proxy setting isn't remembered. When the extension is unloaded, recall this value and set it to what it was prior to the extension being fully loaded.

[jpasski]

Also remember to change the Q&A section of the readme to state what's going on:

A: Yes, by design it modifies the current project-level settings to add upstream proxies. It also will automatically enable "Project options" → "Upstream Proxy Servers" → "Override user options" due to limitations in the Burp Extender API. It currently does not reset this value.