Add a mimetype for Microsoft .ico as used for /favicon.ico

[ssokolow]

I'm using Actix for a little self-contained "run a server off a thumbdrive" thing and I find it a bit grating that the favicon is the only thing where I'm explicitly setting a mimetype and it's stringly typed.

I'm not sure which of the two possible mimetypes would be a better choice:

• image/vnd.microsoft.icon is the officially registered mimetype, but it was submitted by a third party, not Microsoft.
• Outside of favicons, Internet Explorer will only recognize and render .ico files when served as the image/x-icon mimetype that Microsoft was happy to use with no apparent plans to ever register one.