Support custom elements for whitespace preservation

[waterkip]

This change introduces PreserveWhitespaceTags as a configuration
option to support custom elements that may or may not require whitespace
to function.

I'm building webcomponents and some use templates that becomes <pre>
tags. The problem is that the minifier eats my whitespace and thus the
component loose the formatting they require. This is especially annoying
when I format code:

sub foo {
  my @array = qw(here be dragons);
  return \@array;
}

sub bar {
  my @array = qw(go perl);
  return join(" ", @array);
}

And my custom elements gets:

sub foo {
my @array = qw(here be dragons);
return \@array;
}
sub bar {
my @array = qw(go perl);
return join(" ", @array);
}

So I added support to treat custom elements the same as <pre>-tags.

Signed-off-by: Wesley Schwengle wesleys@opperschaap.net