Align artist, album names in candidate listings

Beets currently (1.3.1 from the Debian testing repository) prints tagging prompts in a simple format that's hard to visually scan:

```
1. Alexander Blu - Some of my works from 1995-2005 (69.7%) (tracks, album, missing tracks)
2. Alexander Blu - May (47.8%) (tracks, album, missing tracks, ...) (Digital Media, 2005, XW, Jamendo)
3. Nina Morato - Moderato (45.3%) (tracks, artist, year) (CD, 1999, FR, Polydor)
4. Alexander Ebert - All Is Lost (31.7%) (tracks, album, year, ...) (Digital Media, 2013, XW, Community Music)
5. Blu - Jesus (20.1%) (tracks, album, artist, ...) (CD, 2011, US, Nature Sounds)
```

I would like to see Beets analyze the maximum length of suggested artists and albums, then use sprintf-style formatting to align things in a way that's easier to scan:

```
1. Alexander Blu - Some of my works from 1995-2005 (69.7%) (tracks, album, missing tracks)
2. Alexander Blu - May (47.8%) (tracks, album, missing tracks, ...) (Digital Media, 2005, XW, Jamendo)
3. Nina Morato - Moderato (45.3%) (tracks, artist, year) (CD, 1999, FR, Polydor)
4. Alexander Ebert - All Is Lost (31.7%) (tracks, album, year, ...) (Digital Media, 2013, XW, Community Music)
5. Blu - Jesus (20.1%) (tracks, album, artist, ...) (CD, 2011, US, Nature Sounds)
```