Configuring Shiva

This PR introduces a little more flexibility when configuring Shiva. Now it will scan for config files in this places: 

* ``config/local.py`` relative to the directory where Shiva is installed.
* If an environment variable ``$SHIVA_CONFIG`` is set, then is assumed to be pointing to a config file.
* ``$XDG_CONFIG_HOME/shiva/local.py`` which defauls to ``$HOME/.config/shiva/local.py``.

If all 3 files exist, then all 3 will be read and overriden in that same order, so ``$XDG_CONFIG_HOME/shiva/local.py`` will override any setting defined in ``config/local.py``.