Make shortcut open hovered link instead of current page URL

Added shortcut to launch current page with streamlink: Ctrl+Alt+S. You can change this shortcut using Firefox extension management (https://support.mozilla.org/en-US/kb/manage-extension-shortcuts-firefox)

Fixed API for external calls (see https://github.com/plneappl/streamlink-helper/pull/3 for details)

Added API to let this Add-on be called by other Add-ons. Thanks to freaktechnik (https://github.com/freaktechnik)!
This allows for example his Add-on Live Stream Notifier to open streams without having the user leave his Add-on (https://cup.aihedy.com/en-US/firefox/addon/justintv-stream-notificatio/).

Changed name to appease the reviewer lords.