Add support for pointing resources to a CDN

[Tarmil]

The algorithm to choose what URL to emit for assembly resources and embedded resources is as follows:

• If there is a configuration variable called WebSharper.CdnFormat.<assemblyname>, then use it as URL format.
• Else, if the assembly is part of the standard library and the configuration variable WebSharper.StdlibUseCdn is true, then use the stdlib CDN URL format, which is //cdn.websharper.com/{assembly}/{version}/{filename} by default but can be overridden by the configuration variable WebSharper.StdlibCdnFormat.
• Else, revert to the usual rendering.