Add Eta template engine middleware

[nebrelbug]

As discussed in #5, this PR adds middleware that enables template rendering with Eta 🚀

The middleware works well, but there's one problem. Inside the middleware file, I write

declare module "https://deno.land/x/oak/mod.ts" {
  interface Context {
    render: (fileName: string, data?: object) => void
  }

  interface RouterContext {
    render: (fileName: string, data?: object) => void
  }
}

To add the .render method to the Context type (based on the view_engine middleware). However, this only works if the user is using the same version of Oak; in this case, the latest version. Any thoughts?

I'm open to any feedback or requested changes 😃

[nebrelbug]

I just updated the middleware to include the type declaration on the user's side -- I think in the long run this is best.

[boehs]

it would be awesome if this got merged

[nebrelbug]

I am going to close this PR, since it looks like oakserver/middleware is no longer being maintained.