added listallinfo command

[paulchambaz]

For a personal project of mine, I needed to have quick access to mpd "listallinfo" command.
Since the mpd "listall" command was already implemented, and a Vec structure could already be built from the mpd response, I added the mpd "listallinfo" command. After testing the new command in my project, I had the behaviour I expected.
Let me know if I need to add more code for the pr to be valid.

[naglis]

Thanks!

1. listallinfo (like listall) lists directories, not just songs. Currently, the listall() method is incomplete/incorrect (it does not handle directories, see also issue listall() output is broken #64).
2. This comment should be altered to remove it out of TODO.
3. A test would be appreciated.

[naglis]

Suggested change

	/// lists all songs in the database with metadata
	/// Lists all songs in the database with metadata

[paulchambaz]

1. I'm sorry, but I do not think I am competent enough to add the uri myself. Since listall is incomplete but also present, maybe we could add listallinfo since once one will be fixed, the other will be easily fixed to?
2. I have changed the TODO and will update the pr
3. Again, I'm sorry, I am very new to doing pr, i tried looking for a test on listall to try to test something similar, but I could not find any. Due to the nature of the function, a function that queries the mpd database directly, I'm unsure on the kind of test I could write that would test correct behaviour. It seems to me that both run_command and read_structs should be tested, but I'm afraid I do not have the skill to do such a test.

I have corrected the incorrect docstring and removed the TODO. Again, I'm sorry I cannot be more of use for this project.