Allow turning smartscan to run in all modes save some.

[mwfogleman]

Currently, global-smartscan-mode works in all buffers save the minibuffer.

I find that smartscan is not helpful in shell or eshell-modes. I'm sure there are other modes where it would be useful to turn off smartscan. It would be nice to be able to configure something like

(setq smartscan-ignore-modes '(eshell-mode shell-mode)) and have global-smartscan-mode respect that.

Alternatively, you could also have a function that could be hooked in, e.g. turn-off-smartscan or the like.