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
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
Merge remote-tracking branch 'upstream/master'