Possibility to write spatial metadata to a file instead of return object

[jordansread]

see discussion here DOI-USGS/mntoha-data-release#24 (comment)

simple solution would be to add a fileout arg to meddle::extract_feature(), but it seems like that arg should go first, not last, which probably creates a pile-up of code breaking if we change that. Easier to make that arg last...which I guess does actually match with the pattern in meddle::write_attr_file(), which has file appearing last.

[jordansread]

Idea with ☝️ is that file or fileout being NULL means you just return the object, as normal.