Add `paraboveskip` and `parbelowskip` options

[dcpurton]

It would be useful to be able to set a different above and below skip around the `scripture' environment and its internal environments depending on wither they start a new paragraph or not.

[dcpurton]

Adding parbelowskip is difficult since you can't know easily in advance if there is a blank line after the environment.

(Standard LaTeX lists don't allow setting a different \partopsep below (or a different \topsep for that matter).