Use keyboard shortcuts to paste into active tab

[aanand3]

For Issue #43

One limitation from the docs for chrome.commands:

An extension can have many commands, but may specify at most four suggested keyboard shortcuts. The user can manually add more shortcuts from the chrome://extensions/shortcuts dialog.

This would mean we can't 'reserve' more than 4 shortcuts. Instead, the user would have to go to the dialog to manually assign shortcuts to the different commands, and that page looks like this (note how it will preset/suggest four shortcuts but the rest of the commands are unmapped):

So instead of 5 shortcuts, I can make the extension suggest up to 4 of them. And then add more that can be manually mapped if users want that. Open to suggestions, but I have some ideas!

Got that functionality working and got it to paste into the active tab. Now need to add some mapping between entry and shortcut. More to follow in the coming days!