Set target file fia GET argument

[VSharapov]

Currently the target file is chosen by this line:

readme = 'README.md'

But I'd rather this be a default, overridden by a GET argument like ?target.md=other.md for example

Maybe this could be bundled into settings - with a note not to make it persistent, so you could navigate to a different document by typing into a text field inside the settings widget.