Print Status on SIGINFO

### Feature Request

beets should print out status information on receiving a [SIGINFO signal](https://en.wikipedia.org/wiki/Signal_(IPC)#SIGINFO) (triggered by, for example, the user pressing CTRL-T while beets is performing a long-running database operation).

When I run `beet modify` on a lot of albums, beets takes a few seconds to minutes.
To get information about the current status of the process, I press CTRL-T and get the last line:

```
[...]
Really modify, move and write tags? (Yes/no/select)

load: 0.29 cmd: python2.7 42500 [running] 30.64r 3.05u 14.65s 70% 40900k
```

beets could add it's own output here to show a progress bar or indicator or something like that.

### Setup

* OS: macOS 10.11.6
* Python version: 2.7.13
* beets version: 1.4.2
* Turning off plugins made problem go away (yes/no): haven't tried