Comprehensive relative href/src handling in link rewriter

[awh]

The link rewriter is very brittle at the moment:

• The path segment of relative links must be absolute e.g. /site/...
• Images are expected to in the same directory as the referencing markdown

Relative URLs for both hrefs and images should be able to take any of these forms:

• Full path e.g. /site/features.md
• No path e.g. features.md
• Relative path e.g. ../features.md

any of which can have an optional # anchor appended.