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`
make $items the first parameter in format_items()

The idea is that $items is in fact the most important value, guaranteed
to be mandatory, whereas $format and $fields could be optional.
refactor Post_Command::_list()
make the $fields property private

there really isn't any use for it outside of its own class
Merge branch 'master' into fields-argument

Conflicts:
	php/commands/post.php
Revert parts of 3500a184e5ca652370fdaa7e2c2217290fadf69c.

Changing the order in which arguments are passed will break community use of this helper function.

See https://github.com/wp-cli/wp-cli/commit/3500a184e5ca652370fdaa7e2c2217290fadf69c#commitcomment-3174323
Update post.feature to test new `--fields` argument
Update term.feature to make use of `--fields` argument
Update man docs to mention `--fields`
Merge pull request #408 from wp-cli/fields-argument

A `--fields` argument