Basic basepath support

[rpeyron]

A very simple PR to add support for a basepath of the website, to be removed from pathname before searching/creating GitHub discussion. It adds a data-base-path parameter.

For a jekyll site, it can be simply set to site.baseurl:

       data-base-path="{{ site.baseurl }}"

For development purpose, I have also added the ability to override giscusOrigin to be able to use a custom client.js with the giscus server. I guess this could be useful to other contributors.

[vercel]

@rpeyron is attempting to deploy a commit to the giscus Team on Vercel.

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