add overrideLogo support

recent email thread -- this matches my fork and avoids needless differences.

If you define overrideLogo, that is the URL of the image.
If you define overrideLogoLink, that is the URL of the organization
If you define overrideLogoWidth or overrideLogoHeight, these are the width and height of the image.
If you define overridelogoAlt, that is the alt text (default is 'Logo')

All except overrideLogo are optional.

This overrides prependW3C