3-tier implementation of manifest caching (#57)

* implement manifest caching; refactor config with includes, and generate from ENVs in entrypoint.sh
- disabled by default; enable with -e ENABLE_MANIFEST_CACHE=true
- default times and regexes are a wild guess, make sure to tune for your use case.
-  add manifest caching/anti-ratelimit usage note to README
- add -e ENABLE_MANIFEST_CACHE=true to examples, some wording changes
- add -e ENABLE_MANIFEST_CACHE=true to one the steps in test workflow.

nginx.manifest.stale.conf

@@ -0,0 +1,3 @@
+    # Just like the common block, but adds proxy_cache_use_stale
+    include "/etc/nginx/nginx.manifest.common.conf";
+    proxy_cache_use_stale  error timeout http_500 http_502 http_504 http_429;