Update tweetexploR to work with tidy_tweet's new database schema

This release updates tweetexploR to work with tidy_tweet's new database schema.

Other changes:

• top_n_hashtags() has a new parameter hashtag_source which enables you to analyse the top n hashtags from tweets or user profiles.
• New created_at and retrieved_at date time columns are included in the results of engagement_metrics() which enables users to normalise the engagement metrics by the difference between the time that the tweet was retrieved and the time that the tweet was created.

Publish preliminary tweetexploR to GitHub

Hello and welcome to the preliminary version of tweetexploR! There are several planned features to come (see GitHub issues tagged as 'enhancement'), but for now we hope that the functionality developed so far will be of use to people working with Twitter data collected via twarc and tidied via tidy_tweet.