Fix #962: Allow to pass additional arguments to PhantomJSEnv
Merge pull request #963 from gzm0/phantomjs-args

Fix #962: Allow to pass additional arguments to PhantomJSEnv
Implement System.exit() through environment setting (#958)

This commit removes support for `__ScalaJSExportsNamespace`. Use
`__ScalaJSEnv.exportsNamespace` instead.
Add option to PhantomJSEnv to not immediately terminate (fix #958)