Add doc/examples on style options

[brentfraser]

Expand demo to show color by expression in style def:

    <map-source name="places" type="geojson" title="Cities and Villages" >
        <url>./places.geojson</url>
        <layer name="default">
            <style><![CDATA[
            {
                "fill-opacity": 0.60,
                "fill-color" : [
                  "match",
                  ["get","name"],
                  "Hastings",
                  "red",
                  "Bloomington",
                  "green",
                  "grey"
                ],
                "line-width" : 5,
                "text-field": ["get", "name"],
                "text-allow-overlap": false
            }
            ]]></style>

            <template name="identify" auto="true" />
        </layer>
    </map-source>