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.

test/mocha/unicode.js

@@ -52,7 +52,11 @@ describe("Unicode", function() {
         var tests = [
             'var \\u{0} = "foo";',
             'var \\u{10ffff} = "bar";',
-            'var \\u000a = "what\'s up";'
+            'var \\u000a = "what\'s up";',
+             // Valid ID_Start, but using up 2 escaped characters and not fitting in IdentifierStart
+            'var \\ud800\\udc00 = "Hello";',
+            'var \\udbff\\udfff = "Unicode";', // Same as previous test
+            'var \\ud800\udc01 = "Weird unicode";', // Same as above, but mixed escaped with unicode chars
         ];
 
         var exec = function(test) {