Removed "null" as uninitialized value.
Reviewed poorly designed config validation.

Undefined config vars could just as well me missing from the config list that
couch_config:get/1 returns. On the other hand I don't want to keep track of
that list myself, so I've let get_config throw the code 500 responses.
Moved command generation into get_command/1.
Started to write GET /_ogr2ogr into a plugin test.

At the end of the day, the response should say if either the command is wrong
(because quotation marks are missing or something), or if GDAL_DATA is wrong or
if fallback_crs is wrong.