Neatly wrap multi-line strings

[twpayne]

Currently strings are written on a single line, e.g.

<description>Attached to the ground. Intelligently places itself at the height of the underlying terrain.</description>

It would be nice if these strings were split to some width (e.g. 80 columns) when the output is pretty, e.g.

<description>
  Attached to the ground. Intelligently places itself at the height of the
  underlying terrain.
</description>

This is similar to #5 in that it requires MarshalXML to know whether the indent is not empty.

[twpayne]

This makes for nice human-readable documents, but KML is usually consumed by machines, so closing this for now. Please re-open if needed.