Added two new filters: base_url and admin_base_url

New filters:

* `base_url`: Pass as parameter the `$url` and the `$with_index` variable.
* `admin_base_url`: Same as before, but for the admin base url.

Moreover, I've simplified the `osc_admin_base_url` function getting the first part from the `osc_base_url`.

> P.S.: The rest of the changes of the diff are trailing whitespaces