Support custom warning handlers

https://github.com/emezeske/lein-cljsbuild#custom-warning-handlers

Would probably need handler to resolve symbol to a function, so the handler can be loaded on Cljs pod. Example should show how to call the default handler.

Start with the custom warning handlers example linked in the issue and inspect the current warning-handling entry point. Confirm how a handler symbol should resolve to a function loaded in the ClojureScript pod and how the default handler is invoked. Done means custom handlers work and the example demonstrates calling the default handler.

Written by the indexing model from the issue text.