WIP: Allow using any keys in site configuration and post metadata

[tdemin]

gmnhg currently only handles a handful of the most useful metadata fields, meaning users who want to store additional metadata for use in post templates (tags? taxonomy? out of luck).

This makes gmnhg handle any reasonable property stored in site configuration (with support for gmnhg-specific overrides in the [gmnhg] section) as well as any post metadata property.