progress: Improve the Progress Plugin

[RobLoach]

This change improves the progress plugin by...

• Allows reseting the progress bar to allow running another progress bar after the first one is done
• Replaces the message whitespace so that there's no overlap in messages
• Scopes the variables within the function
• Renames the global variable so that it's namespaced a bit

Here's a good way to test it...

progress 0 "Doing something cool!" && progress 40 "Hello?" && progress 60 "Almost done" && progress 100 "Close" && delay

[akinomyoga]

The message Done! seems to have been dropped. Is this an intentional behavioral change? What is the rationale?