bring in new warnings from Pod::Checker

This is the first set of new warnings for Pod::Simple, imported from Pod::Checker.  I don't expect to supply new ones anytime soon, although I think it would be nice to bring a few more of the Pod::Checker warnings into Pod::Simple.

There are three new warnings:

* if your alt text contains a | or /, like `L<hot/cold|Cold/hot>`
* if the contents of `L<>` starts or ends with space (see below)
* if =over never sees =back by the end of the document

That second one is most interesting to me.  The behavior of L<> between Pod::Parser and Pod::Checker *changes* in the presence of leading/trailing space.  Pod::Checker will assume that `L< foo>` means `L</" foo">` but Pod::Parser would turn it to `L<foo>`!  This warning is particularly useful.

These changes will make my changes to Pod::Checker "complete," and we'll be able to replace Pod::Checker's guts entirely with Pod::Simple.