Improvements to `wp term` output

* Add `--porcelain` argument to `wp term create`
* `wp term create` should include term ID in success message
* `wp term delete` includes the term ID of the deleted term
* Updated tests / doc

See https://github.com/wp-cli/wp-cli/issues/399#issuecomment-16685216
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`
Balance ze curly braces to meet WP standards

https://github.com/wp-cli/wp-cli/pull/407/files#r3886566
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
Simulate and announce a /wp-admin/ page load

Most of the built-in commands need access to code from
wp-admin/includes/ so it makes sense to load it upfront.

Since we're already doing that, it seems like a good idea to also
announce this fact to plugins, by setting WP_ADMIN to true.

And since we're setting WP_ADMIN to true, we don't need to set up the
global $wp_query instance anymore, because the only admin screen where
it's set is the post list screen (wp-admin/edit.php).

See #385
Merge pull request #407 from wp-cli/term-create-porcelain

Improvements to `wp term` output
Define WP_ADMIN before loading WordPress

... where "loading WordPress" also includes loading plugins and themes.

Also, define WP_NETWORK_ADMIN and WP_USER_ADMIN, for completeness.

see #385
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