Add inherited directory header to simplify processing of files

[ajkavanagh]

At present, every file that is to be rendered, needs to have the Yaml header at the top of the file. If creating small files, this becomes a bit of a pain, particularly if I want to automate a 'notes-like' application.

This proposal is that a control file can be added "anywhere" in the site. It will reference a directory (and also allow subdirectories) where there is an implicit header on those files. The control file will also be able to determine the permalinks that the files take up.

Files in that directory can have a header to override the 'default' generated header; e.g. change the link, move it somewhere else, don't render it at all, or keep it in a draft state.