Modified plot.py to pass coords arg through to _map_common()

As highlighted by #448 the coords keyword to ```iris.plot.contouf``` etc. is ignored when the cube is deemed 'mappable'. This PR fixes this issue by modifying ```iris.plot._map_common()``` to use the specified coords if present in ```kwargs```.

~~Note that if a coords keyword arg is not present and multiple x or y coords exist on the cube, the plot methods will still raise an exception as before. If this behaviour is to be changed I suggest it is done in as separate PR.~~ It selects dimension coordinates in preference to auxiliary coordinates if  both are present.

Reviewer: cpelley