Gmail Append HTML [Keyboard]

Gmail HTML Append Extension
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Forked from https://github.com/erichlotto/gmail-html-email
All thanks go to @erichlotto !

This adds the ability to paste via keyboard shorcut, making the whole thing a bit smoother for our use case.

The default keyboard shortcut is 'CTRL+SHIFT+X' - so that you can Cut the HTML (CTRL+X) and immediately paste it as rendered HTML by just adding the shift to the previous keyboard shortcut.