Allow passing function to XMLSerializer.serializeToString for converting entities

[PieterScheffers]

This pull request adds the ability to pass an extra function to XMLSerializer.serializeToString. This function gets the value or data of an TEXT or ATTRIBUTE node and should return with the entities replaced.

The reason is because in my case I needed single quotes and new lines also replaced by ' and 
 respectively in attribute values. But this is not standard compliant I guess. There are a lot of other implementations out there. So if you need to build an XML for such an XML reader it is good when you can replace the entities you want and leave the entities you don't want replaced.

[PieterScheffers]

Example implementation:

const NodeType = {
  ATTRIBUTE_NODE:  2,
  TEXT_NODE: 3
}

const xml = xmlSerializer.serializeToString(doc, isHtml, null, (nodeType, value) => {
  const xmlEncoder = value => {
    switch (value) {
      case '<': return '&lt;'
      case '>': return '&gt;'
      case '&': return '&amp;'
      case '"': return '&quot;'
      case "'": return '&apos;'
      default:
        return '&#' + value.charCodeAt().toString().padStart(2, '0') + ';'
    }
  }

  switch (nodeType) {
    case NodeType.ATTRIBUTE_NODE:
      return value.replace(/[\t\n\r'"&<>]/g, xmlEncoder)
    case NodeType.TEXT_NODE:
      return value.replace(/[&<>]/g, xmlEncoder)
    default:
      return value
  }
})