Customizable left-padding of track and disc numbers

I noticed that importing an album with >99 tracks will cause an inconsistency in left-padding track numbers.

```
'09 Enemy Approaching.opus'
'100 MEGALOVANIA.opus'
'101 Good Night.opus'
'10 Ghost Fight.opus'
'11 Determination.opus'
'12 Home.opus'
```

This also makes ncmpcpp think that Megalovania is the 10th track.

I think beets should make sure that the zero padding scales up beyond >99 tracks.

### Setup

* OS: Arch Linux
* Python version: 3.7.4
* beets version: 1.4.9
* Turning off plugins made problem go away (yes/no): no

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

```yaml
directory: ~/Music
library: ~/Music/beetslibrary.db
import:
write: yes
move: yes
resume: no
match:
distance_weights:
missing_tracks: 0
date: 10
artist: 10
title: 10
length: 10
paths:
default: $albumartist/$original_year $album%aunique{}/$track $title
singleton: $artist/Non-Album/$original_year $title
comp: Compilations/$album%aunique{}/$track $title
```