Adding support for middle, backward, and forward mouse buttons click events, adds config keys : "on-click-middle", "on-click-forward" and "on-click-backward"
Adding the key "format-alt-click" to choose what mouse clicks toggles the alternative format, when present. Possible values (in config): "click-right", "click-left", "click-middle", "click-forward", "click-backward". Other values have the same effect than "click-left". Previous behaviour was to toggle it whenever any click was registered and any click that was not handled by "on-click-right" or "on-click-left" occurred
