GH Meta: support conditional requests

[g-k]

https://snug.readthedocs.io/en/latest/recipes.html#conditional-requests

[g-k]

# TODO: save E-Tag or Last-Modified then send If-Modified-Since or                                                             
# If-None-Match and check for HTTP 304 Not Modified                                                                            
# https://developer.github.com/v3/#conditional-requests                                                                        
# NB: this might not be supported https://developer.github.com/v4/guides/resource-limitations/