Managing labels for multiple issues at once

Allows something like this:
```sh
ghi 1 2 3 4 5 6 -a bug -- LFDM/ghi_test
```

It's multithreaded, the following benchmarks figures are computed by the slowest machine I have around (a dual-core @ 1.3 GhZ)

```
# Multithreaded
ghi label 1 2 3 4 5 6 -a bug -- ghi_test  0,33s user 0,12s system 44% cpu 1,020 total
ghi label 1 2 3 4 5 6 -d bug -- ghi_test  0,32s user 0,11s system 44% cpu 0,988 total

# Singlethreaded
ghi label 1 2 3 4 5 6 -a bug -- ghi_test  0,29s user 0,16s system 10% cpu 4,183 total
ghi label 1 2 3 4 5 6 -d bug -- ghi_test  0,32s user 0,10s system 9% cpu 4,394 total
```

Todo:
- Fix race condition between the mighty Throbbers and regular `puts` statements