Make it possible to specify all source files to parse in conf.py

[jnikula]

With #171 it's now possible to leave out the :file: option as long as the symbol is present in some previously parsed source file, for each rst document. This means the :file: option still needs to be present in some directive.

Make it possible to specify the files to be parsed in conf.py, and only reference the symbols in directives in the rst documents.

See #157, #75 and #156 for previous discussions.