[common] Fix striginfy method

With Python 3 map is a generator function which is not serializable.

The fix is to make it a list.

Fixes #34

beat/cmdline/common.py

@@ -1539,6 +1539,10 @@ def dot_diagram(prefix, type, names, path, formats):
 def stringify(value):
   '''Creates a string representation of a baseformat represented as a dict
 
+  To avoid overloading the terminal that will print the result of this
+  function, only the first ten elements of lists are processed with visual cues
+  that will show that there's more to it.
+
   The input variable ``value`` is modified in-place.
 
   Parameters:
@@ -1569,7 +1573,7 @@ def stringify(value):
       return reduced
     elif (len(value) > 0) and \
             (isinstance(value[0], list) or isinstance(value[0], dict)):
-      return map(stringify, value)
+      return list(map(stringify, value))
 
   elif isinstance(value, dict):
     for name, value2 in value.items():