[feature request] Add generic options for symmetric boxes

[dingens]

In many cases, layouts are symmetric, thus all (left|right)* and (above|below)*, etc. options are set to the same values. It would be nice to have the following settings:

• margin, setting leftmargin and rightmargin
• skip or skipvertical, setting skipabove and skipbelow
• innervmargin or similar, setting inner(top|bottom)margin
• innerhmargin or similar, setting inner(left|right)margin
• maybe innermargin, setting both of the latter (although I guess you rarely need to set them all to the same value, but maybe there are use cases for it; and it would be consistent to have this)

(cf. Figure 2 on page 6 of the documentation. Names are just suggestions, of course.)