Add file extension MIME type mapping

[Loïc Denuzière]

It would be nice to be able to indicate a custom mapping from file extension to MIME type. My current use case is WebAssembly: the browser refuses to load a wasm module if the MIME type isn't "application/wasm".

Maybe with a syntax like this:

dotnet serve --mime .wasm=application/wasm

[Nate McMaster]

LGTM. Let's make a 1.1 release for this.