Added caching to TimberLoader::render

Added transient, site-transient and object cache modes to TimberLoader::render and Timber::render methods. Expiration can differ between logged in and non-logged in users (primarily to disable caching for logged in users). Uses data + template name as the cache key so the same template can be loaded with different data and so that a data change automatically invalidates cache data.
Implemented Twig cache extension

Added Asm89's CacheExtension for Twig using the WP Object Cache, always using Generational Cache Strategy. The key generator generates a key based on all public fields available in passed object or array.
Added TimberKeyGeneratorInterface for classes to generate their own cache key. If array is passed to cache and key '_cache_key' is available, that value is used.
Added more flexible cache mode selection to TimberLoader.

All options are now constants for TimberLoader and default can be set either by extending TimberLoader and overriding $cache_mode, or by adding a filter to timber_cache_mode.
Made cache getter and setter in TimberLoader public

The getter and setter now handle the validation of cache_mode themselves and are more robust overall, so they can be used from outside of TimberLoader.
Made TimberLoader::get_cache and ::set_cache static so you don't need access to an instance to leverage the Timber cache.
Let WPObjectCacheAdapter use the cache getter and setter of TimberLoader.

The cache adapter now accepts a TimberLoader instance instead of a cache mode, so that it simply leverages the default caching setup in TimberLoader
Cleanup and sensible defaults - leave cache off when calling render without setting the expire explicitly, but still allow for cache in templates by default.
Also default to no cache in TimberLoader
Updating docs for new WP.org version
removed extra desc on v0.14
Set caching default to TimberLoader::CACHE_TRANSIENT
Fixed issue where $PostClass for timber-term was not defined
enforcing length on transient keys
merged conflicts with timber-loader->render and timber->render where new hooks for timber debugger were missing
added new hook for timber_loader_render_data after the ksort for Timber Debug Bar