Discard Tab per Sergey Vlasov

Features:

• Alt+U keyboard shortcut
• Tab context menu

This is a fork of Unload Tab.

Changes since the original 1.2 version:

• Fewer permissions
• When unloading the current tab switch to the nearest loaded tab
• Keyboard shortcut changed to Alt+U (can be configured via about:addons -> "Manage Extension Shortcuts")
• Added icon