Command margin colors exposed in settings

This changeset exposes Command Margin color settings in the Default option page of VsVim.

The default black-on-white scheme is blazing on dark themes.
I thought there's nowhere to set the color before I dig into the code, actually.
So I guess it's better to make it more obvious.

About the Filesystem change: sorry I guess it's BOM related.. doesn't compile here so I changed that character. Please ignore that.
Can you revert this part of the change (unless there was some reason for it)?  

IIRC that change exists to help guard against the loading of a specific encoding in the vimrc.