add optional expiry via headers

[rallisf1]

As discussed on #18 (Proposal 1), this was an easy addition.

• [x] If there's an Expires header then a timeout is set which resets the widget and applies the corresponding state and error message
• [x] Fixed the error display to display current error string instead of the generic error string
• [x] Stored the worker to a variable which is already an array for multithreading support
• [ ] Docs reference
• [ ] Tests

P.S. Multithreading needs some brainstorming as you end up with multiple solutions by only splitting the iteration loop.

Feel free to make any changes and don't forget to update the package version.

[ovx]

Thanks for the PR. The multithreading is now implemented in the next branch, it simply splits work between workers and implements an abort controller to stop processing once a solution is found. Both features will be release in the upcoming version.

I'll update the docs and add tests.