sub-budget plotting

Add hooks to specify the specific Budget class to load from the budget module, and to plot specific BudgetItems from the calculated budget.

closes #82 (closed) #70 (closed)

gwinc/__main__.py

@@ -194,6 +194,12 @@ def main():
             print(f'{name} ({type})')
         return
 
+    if args.subbudget:
+        try:
+            budget[args.subbudget]
+        except KeyError:
+            parser.exit(3, f"Error: Unknown budget item '{args.subbudget}'.\n")
+
     out_data_files = set()
     out_plot_files = set()
     if args.save: