Check if undo-redo extension is registered when undoable option is true

[metincansiper]

I think that this extension considers whether undoRedo extension is registered when undoable option is true at some code segments (https://github.com/iVis-at-Bilkent/cytoscape.js-expand-collapse/blob/unstable/src/undoRedoUtilities.js#L2). But it does not consider it consistently, so the extension crashes when you initilize it in undoable mode (it is undoable by default) where the undoRedo extension is not registered and then try to expand/collapse nodes.