Support for read_concatenated_archives option

As discussed in [Issue 348](http://code.google.com/p/libarchive/issues/detail?id=348), here is the first pass at an implementation of the `read_concatenated_archives` option for bsdtar.

Feel free to suggest alternate implementations or significant changes to how this is implemented.  I thought it would be useful to have some code as a starting point for discussion (and so I could see how bad it would be to implement).