Add debug mode

[yardnsm]

The idea is to have a configuration variable, for example: @1password-debug, that when set to 'on', the plugin will:

• Add some extra logging on events.
• Does not exit immediately when an error is thrown.

[BlueDrink9]

If you're after ideas, a branch in my fork has a way to solve this problem.

Annoyingly, because stdout gets captured, getting debug output is tricky. I used a disable-able stderr.