Auto-refer repl-requires as part of connecting

[PEZ]

Calva has a command for requiring REPL utilities, or repl-requires as they are named in clojure.main.

However, CIDER and Leiningen do this automatically. In CIDER's case it is configurable. I don't know if it might be with lein repl too.

CIDER does it like so:

(when-let [requires (resolve 'clojure.main/repl-requires)]
  (clojure.core/apply clojure.core/require @requires))

We should consider doing this in Calva too. And think a bit around wether it should be configurable or not. I lean towards not.

cc: @bpringe @bbatsov @Cyrik