Add --ignore-zeros option to bsdtar

As suggested in #60, this pull request adds support for `--ignore-zeros` as an alias of `--options read_concatenated_archive` for compatibility with GNU tar.

The implementation adds an `option_ignore_zeros` member to `struct bsdtar` which isn't strictly necessary, since `"read_concatenated_archive"` could either overwrite or be appended to `option_options`.  However, (perhaps this is due to my rustiness with C) this would seem to require `option_options` to either be dynamically allocated (in at least some cases - requiring cleanup) or have a static buffer set aside or for `--ignore-zeros` to be incompatible with `--options`, all of which seemed less preferable to me than the additional field in `stuct bsdtar`.  But, if I'm overlooking something or you'd prefer it implemented another way, I'd be happy to rewrite it.

Thanks,
Kevin