possibility to extend parser objects (other than element)

[Jochem Snuverink]

As requested by @nevay. Currently elements can be extended in the parser after the first declaration. It would be nice to have this for other parser objects (fields, tunnels, etc.)

[Jochem Snuverink]

add possibility to extend all parser objects, fixing #218; improve spacing

→ <<cset 3da39645ebc7>>