Include post_type and post_id optionally in the frontmatter; filter attachments

This is rather a heavy commit (so let me know if I should break it out) that does 3 things:

1. it provides a command line option `--include_post_id true` to include the original post_id in the frontmatter. This was necessary for me in migrating my old redirects
2. it adds a `post_type` method on the post migrator and includes the `post_type` in the frontmatter if `post_type != 'post'.` This is helpful for identifying old pages and custom post types from wordpress
3. it filters out `post_type == 'attachment'` which is how Wordpress manages images. The current behavior is to include those attachments as posts in the `export` directory, which is unexpected.

Thanks again. Feedback appreciated.