Add block comment syntax:

{{!-- can contain
      {{handlebars expressions}} --}}

spec/tokenizer_spec.rb

@@ -158,6 +158,18 @@ def tokenize(string)
     result[1].should be_token("COMMENT", " this is a comment ")
   end
 
+  it "tokenizes a block comment as 'COMMENT'" do
+    result = tokenize("foo {{!-- this is a {{comment}} --}} bar {{ baz }}")
+    result.should match_tokens(%w(CONTENT COMMENT CONTENT OPEN ID CLOSE))
+    result[1].should be_token("COMMENT", " this is a {{comment}} ")
+  end
+
+  it "tokenizes a block comment with whitespace as 'COMMENT'" do
+    result = tokenize("foo {{!-- this is a\n{{comment}}\n--}} bar {{ baz }}")
+    result.should match_tokens(%w(CONTENT COMMENT CONTENT OPEN ID CLOSE))
+    result[1].should be_token("COMMENT", " this is a\n{{comment}}\n")
+  end
+
   it "tokenizes open and closing blocks as 'OPEN_BLOCK ID CLOSE ... OPEN_ENDBLOCK ID CLOSE'" do
     result = tokenize("{{#foo}}content{{/foo}}")
     result.should match_tokens(%w(OPEN_BLOCK ID CLOSE CONTENT OPEN_ENDBLOCK ID CLOSE))

src/handlebars.l

@@ -1,5 +1,5 @@
 
-%x mu emu
+%x mu emu com
 
 %%
 
@@ -17,13 +17,16 @@
                                    return 'CONTENT';
                                  }
 
+<com>[\s\S]*?"--}}"              { yytext = yytext.substr(0, yyleng-4); this.popState(); return 'COMMENT'; }
+
 <mu>"{{>"                        { return 'OPEN_PARTIAL'; }
 <mu>"{{#"                        { return 'OPEN_BLOCK'; }
 <mu>"{{/"                        { return 'OPEN_ENDBLOCK'; }
 <mu>"{{^"                        { return 'OPEN_INVERSE'; }
 <mu>"{{"\s*"else"                { return 'OPEN_INVERSE'; }
 <mu>"{{{"                        { return 'OPEN_UNESCAPED'; }
 <mu>"{{&"                        { return 'OPEN_UNESCAPED'; }
+<mu>"{{!--"                      { this.popState(); this.begin('com'); }
 <mu>"{{!"[\s\S]*?"}}"            { yytext = yytext.substr(3,yyleng-5); this.popState(); return 'COMMENT'; }
 <mu>"{{"                         { return 'OPEN'; }