Make supergrep work with Express 4+

[premshree]

Express 4 removes all built-in middleware (except express.static) from its core [1]. This patch updates supergrep to play well with it. I've updated the minimum Express requirement in package.json but I'm not sure how versioning is handled for releases.

I know this project is archived, but... ¯\(ツ)/¯

[1] https://expressjs.com/en/guide/migrating-4.html#core-changes