Added missing comma, which caused Tilemill-error

Resolves the following Tilemill-error:

landcover.mss:239:32 [[feature]=power_substation] added to
[feature]=power_sub_station produces an invalid filter
Merge pull request #551 from floscher/comma

Added missing comma, which caused Tilemill-error
Made some line-colors dependent on their polygon-fill

Objects with the following tags have now a border-color, that depends on
the polygons fill-color:

leisure=swimming_pool/playground/track/pitch
tourism=camp_site/caravan_site/picnic_site
landuse=field/retail
natural=field
power=station/generator/sub_station/substation
amenity=parking
aeroway=aerodrome

The dependency of both colors was expressed by adding:
line-color: saturate(darken(@color, x%), y%)
with 40 <= x <= 60
and 0 <= y <= 30