Adds scaffolding for child theme (based on Twenty Twelve by default)

Apart from creating an empty underscore theme, it would be cool to be
able to create an empty child theme. I've implemented a very basic
command for this. It created a theme folder and style.css which imports
parent theme's styles
Functional test for creating / listing a term
Newline fix for failed test
Actually fix failing tests.

Functions defined inside of functions cause everything to fatal when the parent function is called twice.
Move declaration of compareContents() outside of a function, to avoid fatals when the function is declared twice

See https://github.com/wp-cli/wp-cli/pull/398/files#r3869537
Skip checking these numeric fields, as they might change during the tests

See https://github.com/wp-cli/wp-cli/pull/398/files#r3869915
Simplify expression of the run with errors condition

See https://github.com/wp-cli/wp-cli/pull/398/files#r3869578
rename `scaffold child_theme` to `scaffold child-theme`
make --parent_theme mandatory
style fixes for scaffold child-theme
Land `wp scaffold child-theme`

closes #339
add man page for scaffold child-theme. see #339
wp term list: Support for `--ids` argument
Refactor to allow 'ids' be passed to `format_items()` as a formatting argument
Convert `wp user list --ids` to `wp user list --format=ids`

Also, dropped the formatting of the `$output_items`, as a user is already a WP_User object and the formatting is handled within our `format_items()` util
JSON format test for `wp user list`

Also, fix the CSV used in the setup of the scenario as it originally caused the test to fail
Convert `wp post list --ids` to `wp post list --format=ids`
Update man for `wp post list`
transparently convert --ids to --format=ids

this keeps the back-compat code separate from the command implementation
update `wp post delete` example