Add parameter so text can be optionally not escaped for `tag!`

If I want to add node such as `apples &amp; oranges` without double escaping it, I've to do it with block. such as
```ruby
xml = Builder::XmlMarkup.new({:indent => 2})
xml.root {
    xml.node { xml << 'apples &amp; oranges' }
}
```
but output isn't nice...
```
<root>
  <node>
apples &amp; oranges  </node>
</root>
```
`_indent` is private and so it wouldn't be easy to add indentation myself.

With this PR, I added option to `tag!` that you can specify whether to escape text or not. So we can simply
```ruby
xml.node(false, 'apples &amp; oranges')
```

we get nice result
```
<root>
  <node>apples &amp; oranges</node>
</root>
```

with current implementation, or with `xml.node(true, 'apples &amp; oranges')` or `xml.node('apples &amp; oranges')` it would be double escaped
```
<root>
  <node>apples &amp;amp; oranges</node>
</root>
```