Support 'def', 'defn' and 'fn' optional parameters on signature helper

[fasfsfgs]

As far as I know, clojure doesn't have the notion of optional parameters.

These operations ('def', 'defn' and 'fn') are widely used and their current signature have some optional parameters that are handled inside their implementation. Since the signature helper just looks at the parameter position, sometimes it shows the wrong active parameter (as already noted by @avli in #15).

It would be nice to try to provide better signature helper for them that takes into account more information than just their parameter position.