- Support shift-up/down.

Hi hbin,

I like your concept from `smart-shift` package so I want to contribute something here.

Change:
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* Support shift-up/down for line or region (`smart-shift-up` and `smart-shift-down`).
* Solve key conflict with `cua-mode` (Change `C-c` to `C-C` as a prefix key).
* Add some message to tell user it is in key chord or not.
* Adjust functions and their comments.

TODO:
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* Think about what can `smart-shift` do more. e.g. Maybe `smart-shift` can help to shift number, element in a list or array, codes in the scope, ...etc. It can smartly detect the scenarios and provides options by using `ido` to prompt user to choose.