Specify different horizontal and vertical margins for facets

A straightforward extension. Includes an outstanding update of the .Rd files in the first two commits (i.e., #833), a minor bugfix (8e61554), the implementation with documentation (0001ce5) and an update of the .Rd files with the current changes (f11a4ba). Fixes #839.

No test has been added yet. This code works for me.