Instantiate a Buffer that does not use undo and redo stacks

[dgalaktionov]

I have noticed that, under the current implementation, the undo stack gets appended a copy of the whole text at every new input, which results on unacceptable memory waste for my intended application, in which I plan to edit potentially large (up to 1MB) texts, and will handle undo/redo my own way.

My change simply consists in adding a enable_undo_redo parameter on the instantiation on Buffer and PromptSession (since it always creates its own internal buffer), and using it as a flag to prevent the stacks from being populated in Buffer.undo, Buffer.redo and Buffer.save_to_undo_stack.