Support explicit change outputs

For example:

$ green-cli tx outputs add $ADDR change

Allows specifying a single explicit change output that will act as a
sink for any amount left over after paying the other outputs and fee. In
the absence of an explicit change output the gdk will generate a single
change output.

Author: jkauffman1

green_cli/param_types.py

@@ -32,6 +32,10 @@ def convert(self, value, param, ctx):
             # level. gdk only allows one addressee with send all
             ctx.params['details']['send_all'] = True
             value = 0
+        elif value == "change":
+            # Explicit change output
+            ctx.params['details']['addressees'][-1]['is_change'] = True
+            value = 0
         else:
             value = self.value2sat(value)