Local URL and custom route

This pull request introduces two new features :

1/ Make local URL accessible into globals variable

It was usefull for me to get the local URL of the article into layouts. With the URL, I can populate OpenGraph attributes which need the complete URL of an article.

With this code, the URL is now accessible into ``$globals``, and can be read by layouts.

2/ Add the possibility to set custom function for a given route

I need to be able to create custom function to do custom things outside Textpress.

Now it is possible by defining the ``custom`` key into a route set into config.php.