[rt.cpan.org #21282] Split metafile generation into user overridable methods.

<URL: http://rt.cpan.org/Ticket/Display.html?id=21282 >

I've finally patched this in, sorry for taking so long.  I took a
different route following Module::Build's example.  Rather than have
users override a method, they can supply META_ADD and META_MERGE, like
Module::Build's meta_add and meta_merge attributes, to add/override or
merge in more metadata.  There shouldn't be any need for overriding the
methods.

Rather than repurpose EXTRA_META, it's now gone.

The work you did on the dumper was splendid.  I made a few formatting
tweaks to make the key length stuff a bit smarter, it now figures out a
good length automatically.