Add new `hosts` syntax in the `meta` section to support more extensible data for hosts.

Instead of the old:

	meta:
		default_tags:
			<hostname>:
				- tag1
				- tag2

Use:

	meta:
		hosts:
			<hostname>:
				tags:
					- tag1
					- tag2

You can now specify a `regex` value alongside `tags`, and that will be used
to match against hostnames instead of the <hostname> key. This fixes #2
0.3