GEDCOM extension support

[lorrden]

Implement a system to deal with GEDCOM extensions. GEDCOM extensions are supposed to be prefixed with _.

There are overviews of existing extensions:

• https://www.gramps-project.org/wiki/index.php/Import_from_another_genealogy_program
• http://wiki-de.genealogy.net/GEDCOM/_Nutzerdef-Tag#.C3.9Cbersicht_bekannter_Nutzerdefinierter_Kennzeichen