Refactor CriticalCSS classes

[kovshenin]

• Removes the use of global variables to store Critical CSS options
• Introduces a ->criticalcss pointer to the main base object in modules where needed
• Introduces a autoptimize()->criticalcss() method to return the main base object when needed outside of the CCSS modules
• Introduces ->get_option() and ->flush_options() as a replacement for global variables
• Adds a number of pass-through methods in the base class to access child modules methods (enqueue, log, viewport, etc.)