Make Pattern Library CSS Customizable

[dchukhin]

Description

This pull request makes it possible to customize django-pattern-library CSS by providing a custom CSS file.
Also, the site title is now set by using the SITE_TITLE subsetting (inside of the PATTERN_LIBRARY setting), and still defaults to "Django Pattern Library".

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have added an appropriate CHANGELOG entry