\H file argument toplevel header

To create a separate file with the \H file argument set on the toplevel header, you must put it under the special _file input directory. For example: could contain something like: and it would be associated to the file:

The content of the header = myfile.txt is arbitrary, as it can be fully inferenced from the file path _file/path/to/myfile.txt.bigb. TODO add linting for it. Perhaps we should make adding a header be optional and auto-generate that header instead. But having at least an optional header is good as a way of being able to set header properties like tags.