Add `replace_entities` to the XML PushParser

795cc5a resolved #76 for `Nokogiri::XML::SAX::Parser`. This branch adds the same option to `Nokogiri::XML::SAX::PushParser`.

For example:

```ruby
require 'nokogiri'
include Nokogiri::XML

def main
  before
  after
end

def before
  puts '-'*40, 'before:'
  parser = SAX::PushParser.new(Dest.new)
  parser << doc
  parser.finish
end

def after
  puts '-'*40, 'after:'
  parser = SAX::PushParser.new(Dest.new)
  parser.replace_entities = true
  parser << doc
  parser.finish
end

def doc
  '<e a="x &amp; y">p &amp; q</e>'
end

class Dest < SAX::Document
  def start_element(name, attrs)
    p attrs
  end
  def characters(text)
    p text
  end
end

main
```

Output:

```
----------------------------------------
before:
[["a", "x &#38; y"]]
"p "
"&"
" q"
----------------------------------------
after:
[["a", "x & y"]]
"p "
"&"
" q"
```