Row marks along the base line of objects

[Jeff Kingston]

I have added an option to the next release of Lout which permits the
user to request that row marks go along the baseline of words rather
than at a point half the height of an x above the baseline.

A few people were requesting this because it allows you to align
words on their baselines when their font sizes differ.

The default behaviour remains as before, because this represents quite a
large change in the object model, and I'm concerned that it might degrade
the results of some of the standard packages.  I've changed @Eq so that
it uses the default behaviour even when the surrounding document uses
the new behaviour, for this reason.

Jeff Kingston