Start refactoring lib/Pod/Html.pm

Group all the "global" variables into a hashref $globals.  This variable
will be the return value of init_globals() within pod2html(), will be
passed through, and augmented by, parse_command_line() and then used
through the balance of pod2html().

In essence, so far this is just a renaming of variables.

Remove declarations of superseded variables.

Pass $globals to get_cache(), cache_key() and load_cache().