Add remove-completed command, to remove completed items from a give…

[lennybacon]

Thanks for the nice console application!

I use the app shared with family members and to clean up from time to time I needed a way to remove completed tasks, withot clicking or swiping each item.

So hereby I contribute me addition. I hope you find it useful.

[Regenhardt]

IMO it would be nicer to add an option like --completed-only / -c to the remove command and just add another filter like the [older-than filter] to the existing handler so removal stays in a single command.