Implement a partial billing command

vultron/cmds/billing.py

@@ -0,0 +1,15 @@
+import vultron.cmd
+
+class Billing(vultron.cmd.Command):
+  def vultron_list(self): # == billing history list (lists all payments and voices)
+    """Display all billing history."""
+
+    billing = self.api.get("billing", "history")
+    return billing["billing_history"]
+
+  def vultron_invoices(self): # == billing invoice list
+    invoices = self.api.get("billing", "invoices")
+    return invoices["billing_invoices"]
+
+# billing invoice get <id> is a specific entry from vultron_invoices
+# billing invoice items <id> gets all entries of the invoice