Add a "likes" mode that fetches likes for an account, analogous to query mode

[nadesai]

Intended to resolve #17.

Summary of changes:

• Creates a Mode enum to track whether the script is operating in "users", "tweets", or "likes" mode.
• Creates a new fetch_likes method that uses api.GetFavorites to retrieve tweets liked by a user up to a maximum count.
• Modifies fetch_users to use the new Mode enum and invoke fetch_likes when appropriate.
• Modifies main to construct a Mode enum from arguments and invoke fetch_users appropriately.

[nadesai]

Tested with https://twitter.com/nikhil_a_desai and https://twitter.com/nikhil_a_desai/likes (I liked three random tweets in my feed for the purpose of testing):

$ python fetch_data.py likes nikhil_a_desai
Process query nikhil_a_desai
Found 3/2500 tweets.
Done: no more tweets.
First & last tweet dates are: Sun Jan 30 17:13:49 +0000 2022 - Mon Jan 24 23:47:35 +0000 2022
Found 3 tweets.
Found 3 unique authors.
[0] Fetching friends of @cyrusshahpar46
[1] Fetching friends of @alexnazaryan
[2] Fetching friends of @KimberlyTaylor
Successfully exported 3 nodes to out/nikhil_a_desai/nodes.csv.
Successfully exported 1 edges to out/nikhil_a_desai/edges.csv.

[eleurent]

Many thanks for the great feature!