fix utf8 string handling

substr_replace() is not multibyte safe. Having utf8 encoded characters in the markup, breaks the html output
Limit generated headlines to h6
Merge pull request #208 from apfelbox/max-h6

Limit generated headlines to h6
improve readme
improve readme
improve readme
improve readme
imp
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>
```
Merge pull request #224 from hkdobrev/no-markup-option

Add `noMarkup` option to escape user HTML
improve consistency
fix indents
improve readme
improve extensibility of test case
improve consistency
Ignore html comments as well with markupEscape option.
Add test of escaped html comment.
Merge pull request #236 from naNuke/escapeComment

Ignore html comments as well with markupEscape option.
Merge remote-tracking branch 'upstream/master'