enable autolinking

If set to true, bare links will be automatically turned into urls.
If you write something like `muh http://google.com`, you'll get `muh [url]http://google.com[/url]`, without inserting the angle brackets.

Differences between autolinking and \<autourl>
----------------------------------------------------------
|Markdown | BBCode |
|:----------------------:|:--------------------------:|
|`<https://gist.github.com/alfateam123/7cc354929eb3d8817857>` | `[gist]7cc354929eb3d8817857[/gist]`  |
| `https://gist.github.com/alfateam123/7cc354929eb3d8817857` | `[url]https://gist.github.com/alfateam123/7cc354929eb3d8817857[/url]` |

As you can see, urls that would be recognized as _special links_ (such as gist, wikipedia, deezer...) are not treated specially when autolinking is on. You can get this result, as usual, enclosing the url into `<` and `>`.