Automatically convert URLs into clickable links in posts and pages

[KuJoe]

Properly formatted URLs are detected and converted to clickable links in posts and pages. Ignores URLs already clickable and removes </p> added to some links at the end of lines.

Tested as best I could but I'm sure there will be some cases where this will not work or cause some issues but for properly formatted links this works as expected.

Should resolve #422

[danpros]

This feature should optional because it far more easier to create auto link compared to remove the auto link.

To create auto link we can use theme functions.php and create a custom function there. Example we use it in post.html.php

<?php echo auto_clickable($p->body);?>

[danpros]

The theme functions.php available since version 2.9.1 so it is not yet widely used.

[KuJoe]

Thank you for explaining this! I wasn't aware of the functions.php feature in themes but after doing some more research I was able to figure it out very easily.

I don't like the idea of having to change the code for the existing themes, but it works good. This PR will be closed.