Making Markdown class configurable (see #17)

My suggestion to make Markdown class extendable and configurable so we can override ```beforeProcess``` and ```afterProcess```.

Not ready to merge (no doc). RFC only.

Can be configured like this:

```php
'modules' => [
	'markdown' => [
		'class' => 'kartik\markdown\Module',
		'previewAction' => '/markdown/parse/preview',
		'smartyPants' => true,
		'components' => [
			'parser' => [
				'class' => 'kartik\markdown\Markdown'
			]
		]
	]
]
```