Add IMDB top 5 plot keywords to imdb_lookup

[ianstalk]

Motivation for changes:

This is the highest voted request on Feathub and social distancing is super boring.

Detailed changes:

• Add a imdb_plot_keywords to imdb_parser
• Add required table and FK mappings to the DB/ORM

Addressed issues:

• N/A

Implemented feature requests:

• Feathub https://feathub.com/Flexget/Flexget/+50.

Config usage if relevant (new plugin or updated schema):

This is simply a new field in the entry.

Log and/or tests output (preferably both):

--dump output

-- Accepted: ---------------------------
title            : The Usual Suspects
url              : "http://fake.url"
original_url     : "http://fake.url"
accepted_by      : if
imdb_actors      : {'nm0000286': 'Stephen Baldwin', 'nm0000321': 'Gabriel Byrne', 'nm0001125': 'Benicio Del Toro', 'nm0001629': 'Kevin Pollak', 'nm0000228': 'Kevin Spacey', 'nm0001590': 'Chazz Palminteri', 'nm0000592': 'Pete Postlethwaite', 'nm0000751': 'Suzy Amis', 'nm0002064': 'Giancarlo Esposito', 'nm0000445': 'Dan Hedaya', 'nm0000860': 'Paul Bartel', 'nm0107808': 'Carl Bressler', 'nm0800339': 'Phillipe Simon', 'nm0790436': 'Jack Shearer', 'nm0261452': 'Christine Estabrook'}
imdb_directors   : {'nm0001741': 'Bryan Singer'}
imdb_genres      : [crime, mystery, thriller]
imdb_id          : tt0114814
imdb_languages   : [english, hungarian, spanish, french]
imdb_meta_score  : 77
imdb_mpaa_rating : R
imdb_name        : The Usual Suspects
imdb_original_name: None
imdb_photo       : https://m.media-amazon.com/images/M/MV5BYTViNjMyNmUtNDFkNC00ZDRlLThmMDUtZDU2YWE4NGI2ZjVmXkEyXkFqcGdeQXVyNjU0OTQ0OTY@._V1_.jpg
imdb_plot_keywords: [criminal, suspect, criminal mastermind, flashback, mind game]
imdb_plot_outline: Following a truck hijack in New York, five criminals are arrested and brought together for questioning. As none of them are guilty, they plan a revenge operation against the police. The operation goes well, but then the influence of a legendary mastermind criminal called Keyser Söze is felt. It becomes clear that each one of them has wronged Söze at some point and must pay back now. The payback job leaves 27 men dead in a boat explosion, but the real question arises now: Who actually is Keyser Söze?
imdb_score       : 8.5
imdb_url         : https://www.imdb.com/title/tt0114814/
imdb_votes       : 948269
imdb_writers     : {'nm0003160': 'Christopher McQuarrie'}
imdb_year        : 1995
media_id         : <LazyField - value will be determined when it is accessed>
movie_name       : The Usual Suspects
movie_year       : 1995
original_title   : The Usual Suspects
quality          : <LazyField - value will be determined when it is accessed>
reason           : Matched requirement: "criminal" in imdb_plot_keywords
task             : test

Test output:

platform darwin -- Python 3.7.7, pytest-5.1.3, py-1.8.1, pluggy-0.13.1
rootdir: /Users/ieaston/Flexget, inifile: setup.cfg
plugins: xdist-1.29.0, cov-2.5.1, forked-1.1.3
collected 4 items

flexget/tests/test_imdb_parser.py ....                                                                                                                                                               [100%]

To Do:

• Found some unrelated test bugs I should file issues for.