Prohibit additional chars in unquoted strings.

The general idea here is to permit future syntax extensions;
as a rule, new syntax can only be added by using one of these
characters that can't be a string.

Backtick ` in particular is in the current ECMAScript drafts
for multiline strings, so supporting multiline strings with
that would be a very natural thing to do.

The other characters added here (^, !, ?, @, *, &) are
more or less arbitrary. &, ! and * have special meaning
in YAML so might be natural ones to use for similar
purposes. ? would have some natural "let this be missing"
meaning, @ is often used in template-like things, and ^
could be used for some unforeseen need.

HOCON.md

@@ -188,8 +188,8 @@ A sequence of characters outside of a quoted string is a string
 value if:
 
  - it does not contain "forbidden characters" '$', '"', '{', '}',
-   '[', ']', ':', '=', ',', '+', '#', '\' (backslash), or
+   '[', ']', ':', '=', ',', '+', '#', '`', '^', '?', '!', '@',
-   whitespace.
+   '*', '&', '\' (backslash), or whitespace.
  - it does not contain the two-character string "//" (which
    starts a comment)
  - its initial characters do not parse as `true`, `false`, `null`,
@@ -225,6 +225,10 @@ newline). This rule is from the JSON spec. However, unquoted
 strings have no restriction on control characters, other than the
 ones listed as "forbidden characters" above.
 
+Some of the "forbidden characters" are forbidden because they
+already have meaning in JSON or HOCON, others are essentially
+reserved keywords to allow future extensions to this spec.
+
 ### Value concatenation
 
 The value of an object field or an array element may consist of
@@ -1018,7 +1022,3 @@ Environment variables are interpreted as follows:
  - environment variables always become a string value, though
    if an app asks for another type automatic type conversion
    would kick in
-
-## Open issues
-
- - should a few more special characters be banned from unquoted strings, to allow future extensions?

src/main/java/com/typesafe/config/impl/Tokenizer.java

@@ -228,7 +228,7 @@ final class Tokenizer {
         // chars JSON allows to be part of a number
         static final String numberChars = "0123456789eE+-.";
         // chars that stop an unquoted string
-        static final String notInUnquotedText = "$\"{}[]:=,\\+#";
+        static final String notInUnquotedText = "$\"{}[]:=,+#`^?!@*&\\";
 
         // The rules here are intended to maximize convenience while
         // avoiding confusion with real valid JSON. Basically anything

src/test/scala/com/typesafe/config/impl/TestUtils.scala

@@ -156,6 +156,13 @@ abstract trait TestUtils {
         "[ = ]", // = is not a valid token in unquoted text
         "[ + ]",
         "[ # ]",
+        "[ ` ]",
+        "[ ^ ]",
+        "[ ? ]",
+        "[ ! ]",
+        "[ @ ]",
+        "[ * ]",
+        "[ & ]",
         "[ \\ ]",
         ParseTest(true, "[ \"foo\nbar\" ]"), // unescaped newline in quoted string, lift doesn't care
         "[ # comment ]",