All our documentation for the Azure Service Operator project gets published into the subfolder azure-service-operator and we'd like to use htmltest to catch our broken links.

This PR adds support for this scenario, allowing verification of websites that aren't published to the root of a server.

New configuration BaseURL allows specifying the final URL to which the site will be published.

Any trailing folder that is a part of this URL will be trimmed from absolute URLs before checking to see if the file exists. For example, given BaseURL: http://www.example.com/blog, an absolute reference to /blog/post/2023-05-24.html will have /blog/ trimmed from the start, leaving the filepath post/2023-05-24.html for testing.

New configuration CheckSelfReferencesAsInternal (default: false) allows checking of self-external references as though they are internal.

If you have BaseURL: http://www.example.com/blog and a reference to http://www.example.com/blog/post/2023-05-24.html, instead of failing as a broken link (since the file hasn't yet been published), it will now be converted into the internal reference /blog/post/2023-05-24.html and checked.

Full tests are included.

Closes #45
Closes #47
Closes #184

Some background: We've previously been using different tools to try and verify our markdown prior to rendering by Hugo, but we've had multiple issues with broken links making it through to our live website.