Allow to customize the name of the static folder

This simple hack allows you to change the name of the static folder via a setting in pelicanonf.py. This is a extreme case but sometimes you will need to change the name of the folder.

The commit keeps the default configuration unless the user specifies the following in the settings:

STATIC_URL = 'assets'

Then, the user in its code can do the following. For example in base.html:

<img src="{{SITEURL}}/{{STATIC_URL}}/images/logo.png"/>