Option to disable alternate evil imposed behavior of set-mark-command

Originally reported by: **Sooheon Kim (Bitbucket: [sooheon](https://bitbucket.org/sooheon), GitHub: [sooheon](https://github.com/sooheon))**

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Evil seems to advise set-mark-command in normal state to behave like evil-visual-char, while it retains the emacs behaviour in insert state. If I want emacs style mark selection, I can press `i C-spc` or `\ C-spc`. `C-spc` from normal mode is an exact duplicate of `v`. Would it be possible to add an option to change this behaviour of set-mark-command in normal state to be like `\ C-spc`? It seems wasteful to have two different bindings for the same command, but being able to easily access both the emacsy and vimmy methods of selection is preferable.

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- Bitbucket: https://bitbucket.org/lyro/evil/issue/563

Start by tracing Evil's normal-state binding or advice for set-mark-command, then compare it with the existing insert-state behavior and the \ C-spc binding described in the issue. Done means an option allows normal-state C-spc to use Emacs-style mark selection while preserving the current visual-selection behavior by default.

Written by the indexing model from the issue text.