Note on tests
Helper to test for a project file
Respelling of eterm handling

Includes an explicit test for a file before assuming it is one.

This is similarly implicit behavior (before we were testing if string)
but it's slightly more defensible, I think maybe.
Adding a data loader to guess type based on file extension

It feels more natural to specify a data file by just its name rather
than insisting on a tag. More implicit behavior, but I think justifiable.

File extension mapping is hardcoded for now.
Fix arg name typos
Remove is_list test

Should handle binary and iolists - so let them pass through here.
Handling untagged strings for template content
Fix to hello example

Strings that point to files are treated as file sources.
Leaving in commented alternatives for want of real docs

Sorry universe.