Fix #16 Do not reexport basic types into context

When basic types were reimported to context, it caused problems like

   (identical? (type true) js/Boolean)
   => false

This affected object printing (false was printed as "#<false>"),
incorrect conversion between types (js->clj) etc.

I do not know the original reason for this code, so I am leaving the
rest of the code in place.
Fix #12 Return result as a string

Other repl environments (eg browser-repl) return result as a string. The
change makes cljs-noderepl consistent with other repls.
Fix #18 Pass compiler options to repl-env together with :target :nodejs

Allow caller to pass compiler options. Always add :target :nodejs
so macros like array? are compiled correctly.