rtweet 1.2.0

Authentication changes

• New client_* functions to save, set and use a client app using Twitter API
  via the new authentication mechanism rtweet_oauth2() (see below).
  It has a helper function for the scopes the client is allowed to do set_scopes().
  Endpoint have different scope requirements.

• New authentication mechanism rtweet_oauth2() required by some endpoints.
  Only valid for 2 hours until it is automatically renewed, but it is left to
  the user to save it every time it is renewed.

New endpoints using API v2

New endpoints to retrieve data from twitter, no action is performed (no blocking, muting, posting new tweets...):

• New list endpoints: list_expansions(), list_fields(), list_followers(),
  list_get(), list_members(), list_membership(), list_tweets().

• New tweet endpoints: tweet_get(), tweet_liking_users(), tweet_retweeted_by().

• New user endpoints: user_blocked(),user_bookmarks(),user_by_username(),
  user_following(), user_followers(), user_liked_tweets(),
  user_list_follows(), user_lists(), user_mentions(), user_search(),
  user_self(), user_timeline(), user_tweets().

• New statistic endpoints: tweet_counts_recent(), tweet_counts_all().

Other changes

• Small breaking change: expansions and fields arguments now use NA for all and NULL for none.

• New functions to set expansions and fields for the new functions: set_expansions(), set_fields().

• Fixed a bug that prevented auth_setup_default() to work (#756 and #744)

• Fixed a bug so that the streaming functions use the current token.