Don't run side-effects for pure-clojurescript (non-interop) -> chains

[vemv]

Fixes #30

The introduced logic is as follows:

• If the 'operand' of a (-> ) (i.e. its first member) does not match js/* or js/*, then evaluation (side-effects) will shortcircuited.
• For (. ,,,), (.. ,,,), (doto ,,,), evaluation will never be shortcircuited, because those macros are interop-oriented.

It's not perfect, but it seems to find a balance between shortcircuiting most pure clojurescript forms, and allow JS completions to still work for many cases.

In a future, logic may be refined by leveraging the cljs analyzer - we already do in cider-nrepl so it should be at hand.

Cheers - V

[bbatsov]

I'm fine with the changes, but they'll have to be reflected in the README, so the users don't have a hard time trying to figure out how the completion works.

[vemv]

Thanks!

The readme already happened to say:

This only applies to JavaSript interop code, i.e. JavaScript methods and properties. Pure ClojureScript is not inspected or evaluated.

[bbatsov]

I still think some more examples there won't hurt (e.g. here https://github.com/clojure-emacs/clj-suitable/#how-does-it-work), but I'll leave this up to you to decide if they are needed or not.

[vemv]

Readme expanded - thanks!