Import from paths specified in input file (eg .m3u)

It would be very useful to be able to feed the import command using a file containing paths (for instance an `.m3u` playlist). This could certainly be done with some neat shell-scripting, but would make a useful feature - basically `.m3u` import with the flexibility to help users who want to feed a set of specific folders or songs to beets.

For instance, taking the file `Amandas Summer Songs 2009.m3u`:

```
/home/amanda/music/5ive/singles/Getting Down.mp3
/home/amanda/music/John Cage/misc/4_33_.flac
...
```

the command `beet import -s Amandas\ Summer\ Songs\ 2009.m3u` would import each of those as a singleton (a file listing directory paths could obviously be imported normally, with either an error if a file contains a mix of files and directories or a smart mixed import).

My particular use case is that I have playlists exported from my old iTunes library of ~100,000 songs, divided into playlist by rating. As a result I'm too scared to touch the library for fear of screwing the playlist paths and losing my ratings.

However, by feeding those playlists to beets, I could update the tags on each song, copy them to my current beets library structure, add them to the database, and at the same time generate an `importfeeds` `.m3u` for each import that would reflect the original rating level (and then I could use that with the wonderful new flexible attributes feature to tag them with the appropriate rating).