Caching file system loader

[jg-rp]

This pull request adds CachingFileSystemLoader, a template loader that handles its own LRU cache, instead of relying on the Environment cache. This should improve cache hits when rendering partial templates with {% render %} and {% include %}, and loading parent templates with template inheritance.

from liquid import CachingFileSystemLoader
from liquid import Environment

loader = CachingFileSystemLoader(search_path="templates/", cache_size=500)
env = Environment(loader=loader)
# ...

When initialising an Environment with a CachingFileSystemLoader, the environment cache is disabled and Environment arguments cache_size and auto_reload are ignored in favour of arguments to CachingFileSystemLoader.