Add support for static tile query string args

This application does not support non-plot map features, so we need to
prevent including dots for the non-plot map features on the map tiles.
The tiler supports this with a show= query string argument, so I have
added an OTMEnvironment setting that is appended to the tile url template.
Merge pull request #103 from jwalgran/topic/hide-non-plots

Add support for static tile query string args