In some circumstances, on a reference style image, the last capturing
group is ignored, which causes the fucntion argument to return the number
of matches instead of a string (or undefined).
Checking if the title parameter is a string ensures that the title
parameter is actually something that was caught by the regex and not some
metadata.
Closes#585
One or more spaces before the language declaration of a code block is supported by Github.
E.g.
``` html
<div>HTML!</div>
```
``` html
<div>HTML!</div>
```
```html
<div>HTML!</div>
```
Closes#569
