make `IJavaScriptEnv` `-load` signature consistent, the 2nd argument is goog *provides*
not a single ns

make sure that `opts` is properly threaded through for `cljs.repl/load-dependencies`
and `cljs.repl/load-namespace`

In `cljs.repl/load-namespace` if an `:output-dir` is specified emit `goog.addDependency`
for each dependency computed from the dependecies compilation.

In `cljs.repl/load-file` do a similar thing - emit the `goog.addDependency` for the
file.

For Node.js monkey-patch `goog.isProvided_` to suppress pointless errors about
duplicate `goog.provide` statements for the same namespace.

All built in browsers work.