Make it possible to add remote JavaDoc URLs via CIDER's config

For JavaDoc to work properly for third-party libraries we need to register their JavaDoc like this:

```
(javadoc/add-remote-javadoc "com.amazonaws." "http://docs.aws.amazon.com/AWSJavaSDK/latest/javadoc/")
(javadoc/add-remote-javadoc "org.apache.kafka." "https://kafka.apache.org/090/javadoc/")
```

For the sake of convenience we can add some configuration map that gets evaluated when a REPL gets created, or something along those lines (we already have similar functionality for requiring code on REPL start).

Start by finding the existing configuration used for requiring code when a REPL starts, then trace how that configuration is evaluated during REPL creation. Add an equivalent configuration map for remote JavaDoc entries using javadoc/add-remote-javadoc, and verify that configured library prefixes and URLs are registered when the REPL starts.

Written by the indexing model from the issue text.