Added abstract customizer base class

This will make adding customizer options a bit more streamlined by
taking care of basic sanitization and method loading
Refactored site logo class to use customizer base
Refactored breadcrumb class to use customizer base
Moved breadcrumbs template tag to utility.php

Making this function available at all times allows us to simplify the
logic used in the theme a bit. Without this being always included, we
have to do a function_exists check before using the template tag.
Added a style builder class

Dealing with custom CSS rules controlled by customizer input can be
pretty annoying. This will allow us to streamline the input and safely
print it in the site <head> without re-writing a bunch of complex code
in every theme we create.
Added a helper function for the style builder

This is the function which should be used in themes to interact with
the style builder class.
Added our new files to the includes array
Added logic to sanitize multi-select correctly
Docblock improvements
Split choices and defaults into unique arrays
Made sure site title is displayed by default
Fixed a uri path issue on Windows installations
Improved docs and code readability
Escaped all instances of get_library_uri()

Just to be on the safe side, lets make sure this is escaped before we
use it.
Added support for SVG logos
Version bump
Merge branch 'develop'