fixed getzola#2195

* Reuse Client when checking urls and add timeout for requests

* Implement replace_re filter

* Cargo fmt

* add regex caching

* cargo fmt

* update docs, update unit test

* rename replace_re -> regex_replace

Relative links in the entry content do not currently have a base URI, so
will be resolved relative to the feed URI:

Given an entry with the content:

    <a href="some-resource.bin">

And URIS of:

 * entry: https://example.org/blog/some-entry/
 * feed:  https://example.org/atom.xml

The link URI will end up as:

    https://example.org/some-resource.bin

rather than the URI that ends up resolved in the rendered page:

   https://example.org/blog/some-entry/some-resource.bin

The atom and RSS formats allow for an xml:base attribute (itself
specified in [1]) to provide a base URI of a subset of a document. This
change adds xml:base attributes to each entry, using the page permalink.

This gives us something equivalent to:

    <entry>
     <content xml:base="https://example.org/blog/some-entry/">
      <![CDATA[
       <a href="some-resource.bin">
      ]]>
     </content>
    </entry>

[1]: https://www.w3.org/TR/xmlbase/

Signed-off-by: Jeremy Kerr <[email protected]>