introduce WP_CLI\Formatter class
move disparate formatting utilities into the WP_CLI\Formatter class
let format_items() handle case where fields aren't set, for now
fix get_formatter()
extract find_item_key() method
move logic from format_items() into WP_CLI\Formatter class
use formatter in 'wp role list'
use Formatter in CommandWithUpgrade->_list()
respect --format=json when --field= is passed in list subcommands
set all_with_meta, for WP 3.4
Merge pull request #802 from wp-cli/list-field

`list --field=` ignores `--format=json`