Link rewrite helper command

[preaction]

It'd be nice if, when moving apps and pages around, we could update links quickly using some kind of rule.

A statocles move command might be a good bet. But a statocles relink <old_link> <new_link> or something, where old link gets renamed to new link would also be nice. A statocles relink -i to find broken links and then relink them in a text editor would also be nice.

This could be a case for plugins providing their own commands (#315).