Add g:session_save_color to toggle saving colorschemes.

Allowing toggling the option to save colorschemes and background colors etc with the g:session_save_color option.

This feature can be unwanted when setting colorschemes manually for different situations as in https://github.com/xolox/vim-session/issues/6#issuecomment-10604189 so being able to toggle it on/off would be great.