Per-directory settings files to override global behaviour and make localized settings easier

[jbmorley]

Settings / configuration files are able to pass default values into importers setting, for example, things like the default category for a document. This is incredibly convenient, but configuring it can be a little fiddly as—right now—it necessitates crafting a new regex and importer statement which specifically targets that directory or subtree. It might be much easier to allow per-directory settings files which allow the global behaviours and defaults to be overwritten.