Add an "emptyLines" filter to the preprocessor that strips empty lines from the output. r=Hixie (not part of the build)

config/preprocessor.pl

@@ -447,6 +447,12 @@ sub unfilter {
 
 package filter;
 
+sub emptyLines {
+    my($stack, $text) = @_;
+    $text = "" if $text eq "\n";
+    return $text;
+}
+
 sub spaces {
     my($stack, $text) = @_;
     $text =~ s/ +/ /gos; # middle spaces

config/preprocessor.txt

@@ -177,6 +177,9 @@ Filters
 
 The following filters are supported:
 
+   emptyLines
+     Strips blank lines from the output.
+
    slashslash
      Strips everything from the first two consecutive slash (/)
      characters until the end of the line.