Add `noMarkup` option to escape user HTML

Resolves #106.

This change introduces a new option - `noMarkup`. You could set it the
`setNoMarkup()` method similar to the `setBreaksEnabled()` one.

Example usage:

``` php
<?php

$parsedown = new Parsedown();
$parsedown->setNoMarkup(true);
$parsedown->text('<div><strong>*Some text*</strong></div>');

// Outputs:
// <p>&lt;div>&lt;strong><em>Some text</em>&lts;/strong>&lt;/div></p>
```