For ease of use, allow a CSV of fields to be passed into `format_items()`
Refactor `format_items()` such that it prepares output items in advance, and validates `$fields` based on the fields avaialble on the object.
Add `--fields` support to `wp term list`.

Needs updated tests and doc.
`--fields` support for `wp role list`
Implement `--fields` support for `wp post list`
Implement `--fields` support for `wp user list`
Add empty_blog subcommand to core
Remove some old code [closes #406]
Remove empty_options flag
Use WP_CLI\Iterators\Query instead of ->get_col
Fix post type argument in comments
Change empty_terms and empty_comments to keep_terms and keep_comments
Moving empty blog subcommand to blog command
Move empty blog subcommand to blog command; create separate class for multi-site commands so that we can make blog available to single site installations
Rename wp blog truncate to wp blog empty
Remove empty subcommands args; move empty terms/taxes, posts, and comments; insert default terms/taxes
Update man pages
add basic shell smoke test
shell: use WP_CLI::print_value(), since we don't want to colorize the output
behat: add newline to generated files

`wp shell` expects it