markdown="1" | "Default to Span Mode"

Implements a 'default span mode' in those blocks with the `markdown="1"` attribute that should never have block elements within them. Nothing here fixes the deficiencies mentioned in issue #52, but it does fall under the category of 'better support for markdown="1"'.

The regex in `contain_span_tags` comes directly from php-markdown:
```
# Tags where markdown="1" default to span mode:
protected $contain_span_tags_re = 'p|h[1-6]|li|dd|dt|td|th|legend|address';
```

Spanmode tests:
Input (with 'extra' enabled):
```
<p markdown="1">
    This is not a code block since Markdown parses paragraph
    content as span.
</p>

<p markdown="1">_Hello_ _world_</p>
```

Output Before:
```
<p>

<pre><code>This is not a code block since Markdown parses paragraph
content as span.
</code></pre>
</p>

<p>

<p><em>Hello</em> <em>world</em></p>
</p>
```

Output After:
```
<p>
    This is not a code block since Markdown parses paragraph
    content as span.
</p>
<p><em>Hello</em> <em>world</em></p>
```