`wp <object> get` should support limiting output based off `--fields`

Adds support to `wp post`, `wp comment`, and `wp term`. `wp user` already had it.

Unlike `wp <object> list`, `* get` defaults to all object fields.

Adds `--format=csv` to `* get` as well.
Use https URLs
`wp theme get` and `wp plugin get` should also benefit from 0b729355e905cb561d2dd0d5ea80987f6b384142
Merge pull request #1417 from wp-cli/object-get-fields-1417

`wp <object> get` should support --fields=<fields>