Keep marc4j record for later

Yeah, I know, you're gonna *love* this one.

Some of us -- not you, I know, but some of us -- have code left over from our solrmarc days that take in a marc4j record. It costs essentially _nothing_ to hang onto the original marc4j record and create an accessor for it.

There's a complexity cost, of course -- monkey-patching MARC::Record to have an accessor called `original_marc4j` and yet another settings switch (which I went ahead and called `marc4j_reader.keep_marc4j`).

On the one hand, it's ugly. On the other, I'd really like to get a bunch of uptake, and right now that means getting people to switch over from solrmarc.  On the other-other, it's not very intrusive and unless you turn it on, you'll never know it's there.