Add Co-op status to Retroarch and more player count data to the databases.

Opening this ticket so there'll be a ticket I can reference later after creating a PR. Besides the PSX dat and rdb file we're not tracking the number of players well in any other consoles so at some point I plan to mash up the data against some similar databases that actually contain the player count number and even the co-op status. Although we do not currently have a place to put the co-op status.

I am actually even more interested in tracking co-op status as I am explicitly wanting to try out 2 player games with friends and family. Could we get co-op (simultaneous gameplay) added to the things we track?

+---------------------+---------+------+---------------------+--------------+
| game_title          | players | coop | rating              | release_date |
+---------------------+---------+------+---------------------+--------------+
| Donkey Kong         |       2 | No   | E - Everyone        | 1986-06-01   |
| Bionic Commando     |       1 | No   | E - Everyone        | 1988-12-21   |
| Super Mario Bros. 3 |       2 | No   | E - Everyone        | 1990-02-12   |
| The Legend of Zelda |       1 | No   | E - Everyone        | 1987-08-22   |
| Kirby's Adventure   |       1 | No   | E - Everyone        | 1993-05-25   |
| Metroid             |       1 | No   | E - Everyone        | 1987-08-15   |
| Mega Man 5          |       1 | No   | E - Everyone        | 1992-12-15   |
| Kid Icarus          |       1 | No   | E - Everyone        | 1987-07-01   |
| Lemmings            |       1 | No   | E - Everyone        | 1991-02-14   |
| Castlevania         |       1 | No   | E10+ - Everyone 10+ | 1987-05-01   |
+---------------------+---------+------+---------------------+--------------+

Start by comparing the PSX dat and rdb files, then inspect how other console databases represent player counts. Determine where a co-op field could be stored and how matching data would be imported. Done means the database format supports co-op status and player counts consistently across the targeted consoles.