added custom casts - CleanHtml, CleanHtmlInput, CleanHtmlOutput

[tabacitu]

Fixes #152

Adds three custom casts:

use Mews\Purifier\Casts\CleanHtml;
use Mews\Purifier\Casts\CleanHtmlInput;
use Mews\Purifier\Casts\CleanHtmlOutput;

This way, the dev can more easily use HTMLPurifier directly inside their Eloquent models, without writing an accessor or mutator.

I'll follow up with a different PR for documentation.