Expand relative paths in href and src attributes via replace-match in sanitize-html function.

[c1-g]

Addressing #41 and maybe #45 too?

All the work is done in org-web-tools--sanitize-html.

Added a new variable called org-web-tools-expand-relative-path.
If it's nil, relative paths won't be expanded.

This works by searching the temporary html buffer created by org-web-tools--sanitize-html
for all href and src attributes, pass the value of each attribute to url-expand-file-name,
with the URL argument or the url in user's kill ring being its base, replace the value of the
attribute with the result.

I tested this on a handful of Wikipedia articles and it works just fine.

[c1-g]

After a few more testing the expanding function raised error
(error Invalid use of ‘\’ in replacement text)
when it tried to expand

<a href="#Preorder_R\R">

in https://en.wikipedia.org/wiki/Binary_relation

Maybe quoting the replacement text will resolve the issue.
I'll close this PR until the problem is resolved.

[alphapapa]

Hi,

Apologies for overlooking this PR for so long.

Looking at #41 again, and based on what I've learned since then, I think the best way to solve this issue would be to parse the HTML to a DOM object with libxml-html-parse-buffer, then walk the DOM using the dom library and modify any anchors' HREFs accordingly. Then the DOM can be serialized back to HTML using shr-dom-print. That should be more robust and reliable than using regexp matches on the HTML.

Also, I think the --sanitize-html function is not the place to do this change; its purpose is to "sanitize" the HTML, i.e to make it clean and safe, and adjusting links is a different purpose.

So, would you like to adjust this PR accordingly? If you're not interested anymore, that's fine, too. Just let me know.

Thanks.