Add optional check for experiment cookie to persist previously seen variations (Fixes #93)

[alexgibson]

Whilst traffic cop no longer sets cookies itself, there are circumstances where we might want to ensure a variation is seen again on repeat page visits once a visitor is entered into an experiment (such as a full page redesign).

This PR adds an optional check to read an experiment cookie that can be set by the initiating website. This enables websites to be responsible for handing cookie consent, and provided cookies are accepted, Traffic Cop can then more consistently show the same variation on repeat visits.

Testing:

1. npm install && npm run build
2. cd demo && npm install && npm start
3. Open http://localhost:3030

• Verify that each of the three demo pages show the same variations on repeat navigations to the base page.