Merge configs to allow partial key/value overrides

[seothemes]

Currently, the Child Theme Lib looks for a complete config file in the child theme. If one is not found it falls back to the default config.

A better approach would be to allow partial config overrides by merging the default config with the child theme's config - meaning that the child theme only needs to provide keys/values which differ from the default.

Example of proposed functionality:

$default = [
	'colors' => [
		'link'   => '#333333',
		'button' => '#dd3333',
		'accent' => '#dd3333',
	]
];

$override = [
	'colors' => [
		'link'   => '#555555',
	]
];

The above configs will return the following:

$config = [
	'colors' => [
		'link'   => '#555555',
		'button' => '#dd3333',
		'accent' => '#dd3333',
	]
];

However, in the current version it will only return the override:

$config = [
	'colors' => [
		'link'   => '#555555',
	]
];

This is not ideal because it requires child theme's to provide an entire config, even if there is only one difference from the defaults.