dropdownToggle: add support for Escape key closing an open dropdown

[ispringer]

It would be nice if the Escape key closed an open dropdown, as it does by default for modals created via the modal directive. That way dropdown menus could be both opened and closed using only the keyboard.

[chrisirhc]

Just adding a note here that this is part of the default behavior of the dropdown on Bootstrap and as such we may want to consider adding this.