Add ability to perform 'duplicate up'.

Hi,

I was going to submit my own plugin to Package Control, but decided to try sending you a pull request first.

Here's my plugin:
https://github.com/harawata/sublime-duplicate-line-eclipse
As shown in the README, your plugin has __Copy Lines__ behavior already and I would like to add __Duplicate Lines__ behavior.
Please see the .sublime-keymap file for how to assign key bindings.
This change should be backward compatible.

What do you think?

Best regards,
Iwao