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
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
Remove empty subcommands args; move empty terms/taxes, posts, and comments; insert default terms/taxes
Initial implementation of wp media import

Introduces `wp media import` command to sideload images and import
them as attachments. Needs tests, second opinion, etc.
Add capability to download files from URL

If a recognizable scheme is passed in the <file> argument, `wp media
import` will attempt to download the file to a temp directory before
importing it.
Remove filename completion for wp media import

Filtering bash completions breaks bash's default filename globbing,
which is a more useful feature here.
Rename arguments to make more logical sense

Use the same field labels as are used by the admin media uploader, to
avoid confusion: title / caption / alt / description; rather than the
variable names used internally, where 'description' is really title and
so on.
Add Behat feature for `wp media import`