Hooks, better path filtering, removal of superseded config

• Added hooks! Check out how to use them in the README, they let you configure things and "hook" Conjure into other tools like REBL. An example can be found in the README's hook section.
• Breaking: Removed the :refresh config from .conjure.edn, you can now use the :refresh hook to configure the exact same thing.
• Added :dirs config, an optional Clojure set that limits connections to specific directories. This should come in handy for monorepos etc.
• Added :extensions config, an optional set that defaults to #{"clj" "cljc" "edn"} for Clojure and #{"cljs" "cljc" "edn"} for ClojureScript. This can be used to add or remove certain file extensions for your connections.
• Added :exclude-path? config, an optional function that returns true or false for each file path you might want to consider for evaluation for a connection. This can be used for custom filtering of paths for connections when the previous two don't quite cut it.
• Breaking: Removed the :expr config since it's been superseded by the three previous tools. If you'd like to just use regex filtering you can do that via :exclude-path?.
• Tweaked some launch options for Conjure's internal JVM to prevent warnings with JDK 13.

I hope you find these changes useful!