Support Tumblr API

- $RUBOTY_TUMBLR_CLIENT=tumblr to use tumblr api always
- $RUBOTY_TUMBLR_CLIENT=google to use google api always
- $RUBOTY_TUMBLR_CLIENT=hybrid to use tumblr api on unsafe requests
(default)

Prior to use tumblr API, you have to set $RUBOTY_TUMBLR_API_KEY