KML styling issues

Hi,
I'm using kml in cesium and I have some problems:
1. when I'm using external file as the styling source the kml is not loaded.
For example I have 2 files:
## example.kml

```

example.kml


entityName
style.kml/#0

10.04, 25.8907

```

and
## style.kml

```

style.kml


normal
#n0


highlight
#h0



<IconStyle>
<Icon>
<href>arrow.png</href>
</Icon>
</IconStyle>


<IconStyle>
<Icon>
<href>arrow.png</href>
</Icon>
</IconStyle>

```

The `example.kml` is not loaded in cesium.
When I add the style tag into example.kml and change the href attribute- the file is loaded properly.

It would be great if cesium will support this ability but in any case I think that if cesium is failed to load style or every other attribute that is not supported yet (`ExtendedData` for example), it should silently ignore the problem and apply the default styling/settings.

Reproduce the failure using example.kml and its external style.kml, then trace Cesium's KML loading and styling entry points. No source file or test is named in the issue, so locate the relevant parser coverage first. Done means external StyleMap references no longer prevent loading, while unsupported styling or ExtendedData is ignored and default styling is applied.

Written by the indexing model from the issue text.