Save the dashboard instance as a global variable

This allows custom commands to make use of the dashboard instance, see:

https://github.com/cyrus-and/gdb-dashboard/wiki/Develop-custom-commands

This should ease the adoption of custom dashboard Python commands like the one
suggested in #257.

Author: cyrus-and

.gdbinit

@@ -574,6 +574,8 @@ class Dashboard(gdb.Command):
 
     @staticmethod
     def start():
+        # save the instance for customization convenience
+        global dashboard
         # initialize the dashboard
         dashboard = Dashboard()
         Dashboard.set_custom_prompt(dashboard)