* Add 'filepath' source attribute to tiddlywiki.files
Adds a new source attribute to facilitate importing subdirectories with
the `searchSubdirectories` attribute.
The `filepath` atttribute is like `filename` except it includes the full
pathname relative to the directory path. Consequently it only works with
`directories` sections (not `tiddlers`).
* Add `subdirectories` source attribute to tiddlywiki.files
Adds another new source attribute to facilitate importing subdirectories
with the `searchSubdirectories` attribute.
The `subdirectories` source attribute is only usable with `directories`
sections (not `tiddlers`). It resolves to an array containing all the
directories in the file's path relative to the directory path. Eg, if
the directory path is `files`, `{ source: 'subdirectories' }` for a file
in `files/images/photos` would resolve to `['images', 'photos'].
This commit also adds an example to the "tiddlywiki.files Files"
documentation tiddler that demonstrates using the `filepath` and
`subdirectories` source attributes to import and auto-tag image files
as external-image tiddlers.
