Beets: Rename the update command

[sampsyo]

The beet update command name often (very often!) fools users into thinking that it performs a re-import or does what the mbsync plugin does. This is understandable: it sounds like it _updates_ the information from MusicBrainz.

I think it's time to rename the command. My current favorite name is read, since it's the opposite of the write command. Other suggestions have included resync and refresh.

We would of course keep update as an alias but leave that out of the documentation.

[geigerzaehler]

:thumbsup: for "read".