Dev/Prod mode?

[nickguimond]

https://github.com/lucacasonato/fresh/blob/95289380f3f8bb85c8cc2d0fc3e5ab2ab4de9962/src/server/context.ts#L39

Dev/Prod mode is determined by assuming the end user is using Deno Deploy. I just set a random string for now in my use case but would be nice if this was a config option or something agnostic to the deployment environment. Or even just something to mention how to manually override in the docs.