Do not fail on unknown -X flags

For better compatibility if we add experimental stuff in 1.0.X compilers

compiler/cli/cli-common/src/org/jetbrains/kotlin/cli/common/arguments/CommonCompilerArguments.java

@@ -58,6 +58,8 @@ public abstract class CommonCompilerArguments {
 
     public List<String> freeArgs = new SmartList<String>();
 
+    public List<String> unknownExtraFlags = new SmartList<String>();
+
     @NotNull
     public String executableScriptFileName() {
         return "kotlinc";