Vanity URL

[tj]

• serve a default origin when mapping is incorrect etc.. CF needs one anyway
• serving from mapping
• missing slug page
• api for mapping
• integrate into up(1), up url etc
• inject favicon if missing
• inject "Powered by Up" stuff because if you're using the vanity url it's obviously a sideproject / in development, and I have to pay for CloudFront traffic :D
• write blog post... CDN... why people should use it over others