Add support for term meta data

[stephenharris]

WordPress 4.4 adds term meta data, and trac ticket 34602, adds term meta data to the WXR export file. Although that ticket is still open, and is unlikely to be in 4.4, this PR adds support for importing term meta data, following the schema set out in that ticket.

This PR replaces #18

[stephenharris]

If wxr_importer.pre_process.term_meta returns a false-y value all subsequent term meta is skipped, not just the current one. This doesn't seem to me to be the intended behaviour, but is consistent with how the corresponding hooks for post meta and comments currently work.

If that is indeed a bug then I'm happy to open another PR to fix that, and modify this one accordingly.

[rmccue]

Agreed, I think it should be a continue not a return