Don't allow escaped surrogated identifiers + introduce ascii_identifiers

Don't use 2 characters for surrogates in identifiers because there is
support for the \u{} syntax when escaped identifiers were introduced.

Also catch eof errors while reading identifier names

Introduce ascii_identifiers:

By setting ascii_identifiers to undefined (default value),
ascii_identifiers will print identifiers using the same setting as
ascii_only within the limits of the ecmascript 6 grammar.

ascii_identifiers accept true and false, allowing identifiers to be
printed under different settings than strings with the ascii_only setting.

lib/output.js

@@ -53,6 +53,7 @@ function OutputStream(options) {
         quote_keys       : false,
         space_colon      : true,
         ascii_only       : false,
+        ascii_identifiers: undefined,
         unescape_regexps : false,
         inline_script    : false,
         width            : 80,
@@ -70,6 +71,8 @@ function OutputStream(options) {
         keep_quoted_props: false,
         ecma             : 5,
     }, true);
+    if (typeof options.ascii_identifiers === 'undefined')
+        options.ascii_identifiers = options.ascii_only;
 
     var indentation = 0;
     var current_col = 0;
@@ -81,8 +84,11 @@ function OutputStream(options) {
         return str.replace(/[\ud800-\udbff][\udc00-\udfff]|[\u0000-\u001f\u007f-\uffff]/g, function(ch) {
             var code = get_full_char_code(ch, 0).toString(16);
 
-            if ((identifier && code.length === 1 && !options.es5) || code.length > 4) {
+            if ((identifier && code.length === 1 && options.ecma >= 6) || code.length > 4) {
                 if (options.ecma < 6) {
+                    if (identifier) {
+                        return ch; // no \u{} support
+                    }
                     return "\\u" + ch.charCodeAt(0).toString(16) + "\\u"
                         + ch.charCodeAt(1).toString(16);
                 }
@@ -165,7 +171,7 @@ function OutputStream(options) {
 
     function make_name(name) {
         name = name.toString();
-        if (options.ascii_only)
+        if (options.ascii_identifiers)
             name = to_ascii(name, true);
         return name;
     };