Add --target esnext

Currently, this disables the rest and spread transforms. This will
change as proposals enter and leave stage 3.

src/compiler/commandLineParser.ts

@@ -265,6 +265,7 @@ namespace ts {
                 "es2015": ScriptTarget.ES2015,
                 "es2016": ScriptTarget.ES2016,
                 "es2017": ScriptTarget.ES2017,
+                "esnext": ScriptTarget.ESNext,
             }),
             description: Diagnostics.Specify_ECMAScript_target_version_Colon_ES3_default_ES5_or_ES2015,
             paramType: Diagnostics.VERSION,