Extend mbsubmit plugin to run after initial import

[arsaboo]

Description

Adds a command mbsubmit to run even after the initial import. The command accepts queries and prints the tracklist to be submitted to MB. Users can now run something like beet mbsubmit album:"Atrangi Re" to generate the track listing (see output below).

arsaboo@ubuntuvm2:~/beets$ beet mbsubmit album:"Atrangi Re"
01. Garda - A.R. Rahman, Daler Mehndi (3:44)
02. Chaka Chak - A.R. Rahman, Shreya Ghoshal (4:31)
03. Tere Rang - A.R. Rahman, Haricharan, Shreya Ghoshal (4:34)
04. Little Little - A.R. Rahman, Dhanush, Hiral Viradia (4:18)
05. Tumhein Mohabbat - A.R. Rahman, Arijit Singh (3:07)
06. Rait Zara Si - A.R. Rahman, Arijit Singh, Shashaa Tirupati (4:51)
07. Toofan Si Kudi - A.R. Rahman, Rashid Ali (3:37)

Fixes #4455

To Do

• Documentation. (If you've add a new command-line flag, for example, find the appropriate page under docs/ to describe it.)
• Changelog. (Add an entry to docs/changelog.rst near the top of the document.)
• Tests. (Encouraged but not strictly required.)

[sampsyo]

Wow; excellent!! Nice work on this—it's a clean and obviously useful addition. 👏

[JOJ0]

Awesome feature @arsaboo ❤️