[CHANGE] Warnings about deprecated elements

[Rob Beezer]

An unexpected side-benefit of careful markup is that we can tell exactly when
you are using old elements that we had to deprecate at some point, and tell you.

When you build your entire project (and not when you do things like generate
assets) we have limited the time period for which deprecated elements get
checked to a two-year look-back by default. We thought this process of always
checking very old deprecations was an inefficient use of time, when it really
only uses less than 1% of the build-time. So, a few changes:

* We only provide informational messages when we actually do any checks at all
(a bug fix).

* The default is now to check *all* deprecations from all time. (We have been
averaging about 10 a year, for ten years.) Until now, we defaulted to checking
just the last two yeatrs. We are not deprecating the parameter and replacing it
with a new one! Just changing behavior.

* But if your project is big, and well-maintained, you can elect to have a
"limited" time look-back, by setting a string parameter
"author.deprecations.all" to "no" ("yes" being the new default).

* Since the time consumed is not so bad, I have set the "limited" time to four
years (from its previous value of two years).

* Coming soon in the next release of the CLI, but you can certainly set the
string parameter to your preference now.

Thanks again to Andrew Scholer for putting this scheme in place originally.

Rob