Move container selector into a property

[said-moj]

• Move container selector into a property so that it can overridden when needed without modifying the script
• Make timeout warning message as properties so that they can be overridden without changing file

[hannalaakso]

Hi @said-moj

Thanks a lot for your PR. I've been working on updating the component to use GOV.UK Frontend and have moved the fork to use that instead of prototype kit. See https://github.com/hannalaakso/accessible-timeout-warning/commits/update-component

I'm going to merge those changes into master and then have a look at yours, they look broadly sensible to me 👍

You can find me on cross gov slack if you want to chat about any of this.