don't escape relative link anchors

[zkamvar]

This will address #85 by not escaping relative link anchors if they have escapable characters.

Since these are not rendred by markdown to HTML, there is no reason to escape them, thus this pattern will now work:

here is a [linke that we use an underscore in][underscored_link]. It will not
be escaped when we roundtrip with {tinkr}!

[underscored_link]: https://github.com/ropensci/tinkr/issues/85

[maelle]

🚀 especially as this touches the xsl file!