Add support for linting `alt`/ `title` attributes

We currently ignore all HTML tag attributes but, unlike the others, `alt` and `title` actually contain real text.

Some examples:

###### Markdown

```
![Logo](/images/logo.png)
```
###### AsciiDoc 

```
image:logo.png[Logo]
```

###### reStructuredText

```
.. image:: /images/logo.png
   :alt: Logo
```

