Allow exception handling to be disabled

The default exception handling is very inconvenient while developing and testing a controller as it may hide difficult to spot mistakes behind a very generic 500 error. A solution proposed in #5 would help in development, but not when writing tests.

An afterthought: `handle_exceptions` is very similar to the `handled_exceptions` attribute and `handle_exception` method of `Lotus::Action::Throwable` so it may cause unintentional problems with typos. I'll gladly fix the PR if any better name is suggested.