Use symlinks for (some) redirects

[wincent]

At the moment we support various kinds of redirects via frontmatter metadata.

"wikilink" style redirects that link to a named article:

redirect: [[Abstract Syntax Tree]]

Redirects to absolute URLs:

redirect: https://github.com/wincent/ferret

Redirects to relative URLs:

redirect: /forums/

It would be nice if we could also have redirects via symlink for linking to named articles; eg.

cd content/wiki
ln -s "Abstract Syntax Tree.md" AST.md 

Obviously would need to sanity-check these (ie. link target would have to be in the content/ subtree).