bpm plugin results not reflected in file metadata

### Problem

After using the bpm plugin, the results are stored in the beets library but not in the files themselves. I couldn't find any information about why this should be the case.

I have recently installed beets and tried the bpm plugin. I imported some of my music which was copied over to `~/Music/Radiohead`

My config file originally just contained the reference to the plugin, after which I typed `beet bpm Prove Yourself` at the cmd line. The resulting bpm was added to the song to beets library (which I confirmed using the command `beet ls -f "bpm is $bpm" Prove Yourself`. Winamp shows an empty bpm tag in id3v2 and under basic info.

I thought I may need to update the bpm so I ran `beet update` which suggested the change from 110bpm to 0bpm, which it was able to write successfully (essentially undoing my hard work of tapping enter 3 times to get the bpm). So I guess updating here prefers whatever is currently written in the file over what's in the beets library?

I then tried modifying the config file to what what you see at the bottom of this post but there was no change in behavior. Am I doing something wrong?

Running this command in verbose (`-vv`) mode:

``` sh
C:\Users\Jared>beet -vv bpm Prove Yourself
user configuration: C:\Users\Jared\AppData\Roaming\beets\config.yaml
data directory: C:\Users\Jared\AppData\Roaming\beets
plugin paths:
Sending event: pluginload
library database: C:\Users\Jared\AppData\Roaming\beets\library.db
library directory: C:\Users\Jared\Music
Sending event: library_opened
bpm: Press Enter 3 times to the rhythm or Ctrl-D to exit

Sending event: database_change
bpm: Added new bpm 110
Sending event: cli_exit
```
### Setup
- OS: Windows 8.1
- Python version: pip linked to 2.7.9, but cmd line linked to 2.7.10 :: Anaconda 2.3.0 (64-bit),
- beets version: 1.3.17

My configuration (output of `beet config`) is:

``` yaml
import:
copy: yes
move: no
write: yes

plugins: bpm
bpm:
max_strokes: 3
overwrite: yes
```

Thanks for the software by the way.