Added character set mangle options (#3436).

These currently do nothing, but will soon define which characters the mangler is
allowed to use when generating names.

README.md

@@ -783,6 +783,13 @@ If you're using the `X-SourceMap` header instead, you can just omit `sourceMap.u
 - `toplevel` (default `false`) -- Pass `true` to mangle names declared in the
   top level scope.
 
+- `leading_character_set` (default `abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ$_`) -- Pass
+  a string of characters to be used at the start of, within, or at the end of
+  mangled names.
+
+- `trailing_character_set` (default `0123456789`) -- Pass a string of characters
+  only to be used within, or at the end of mangled names (never the start).
+
 Examples:
 
 ```javascript

lib/scope.js

@@ -375,6 +375,8 @@ function _default_mangler_options(options) {
         keep_fnames           : false,
         reserved              : [],
         toplevel              : false,
+        leading_character_set : "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ$_",
+        trailing_character_set: "0123456789"
     });
     if (!Array.isArray(options.reserved)) options.reserved = [];
     // Never mangle arguments