only format if a tag comes back when peeking

this patch addresses the following problem where

```html
[<a href="http://jsbeautifier.org">beautify</a> js and more]
```

gets formatted as

```html
[
<a href="http://jsbeautifier.org">beautify it</a>js and more]
```

you'll notice that before the <code>&lt;a&gt;</code> there is a newline and after the <code>&lt;a&gt;</code> a space is removed. 

this happens because <code>get_tag()</code> when peeking (in order to support HTML5 A block mode) returns <code>beautify it&lt;a&gt;</code> and sets that value to <code>next_tag</code>.

thus, <code>this.Utils.in_array(next_tag, unformatted)</code> will now return false, causing the entire <code>&lt;a&gt;</code> code block to be treated as a formatted tag (even though by default <code>&lt;a&gt;</code> is an unformatted tag).

this patch now inspects <code>next_tag</code> properly. if it comes back as a valid HTML tag, then it respects the unformatted rules, otherwise if a non HTML tag (ex: text content) comes back, then it gets treated as as unformatted block.

with this change, it preserves spacing and the above example will properly return

```html
[<a href="http://jsbeautifier.org">beautify</a> js and more]
```

after being parsed. since the test framework does not seem to support HTML beautification, i did not submit tests with this PR.