Places an map on the stack.
This function filters the map (original). Any item that does not contain the specified filterkey and filtervalue will be removed.
For example, if you had the following map in your stack…
[ { "name": "alpha",
"colour": "orange",
"style": "plain" },
{ "name": "beta",
"colour":"blue",
"style":"exotic"} ]
…then the command 'colour' 'blue' FILTERMAP will remove 'alpha' from your map, leaving only 'beta'.