Allow more characters in header IDs

The current version only allows characters as defined by HTML4. However,
HTML5 allows any characters to be in an ID. Since allowing any character
might not play well with the XML syntax of HTML5, extend the allowed
character set to the full set allowed by XML.

lib/kramdown/parser/kramdown/header.rb

@@ -8,6 +8,7 @@
 #
 
 require 'kramdown/parser/kramdown/block_boundary'
+require 'rexml/xmltokens'
 
 module Kramdown
   module Parser
@@ -40,7 +41,7 @@ def parse_atx_header
 
       protected
 
-      HEADER_ID = /[\t ]{#(?<id>[A-Za-z][\w:-]*)}\z/
+      HEADER_ID = /[\t ]{#(?<id>#{REXML::XMLTokens::NAME_START_CHAR}#{REXML::XMLTokens::NAME_CHAR}*)}\z/
 
       # Returns header text and optional ID.
       def parse_header_contents