MINOR: map: add map_*_key converters to provide the matching key

MINOR: map: add map_*_key converters to provide the matching key

All map_*_ converters now have an additional output type: key. Such
converters will return the matched entry's key (as found in the map file)
as a string instead of the value.

Consider this example map file:
 |example.com value1
 |haproxy value2

With the above map file:

str(test.example.com/url),map_dom_key(file.map) will return "example.com"
str(running haproxy),map_sub_key(file.map) will return "haproxy"

This should address GH #1446.