Add support for S24_3LE format

I'm not sure if libsoundio is to blame, but my external sound card is supposed to support 24bit/96kHz (Fiio E10, has reviews so not obscure to Windows/Mac users). The C equivalent of what's failing is:

```
...
SoundIoOutStream *out = soundio_outstream_create(device); // not null for me
out->sample_rate = 96000;
out->format = SoundIoFormatS24LE;
int err = soundio_outstream_open(out)); // soundio_strerror is "unable to open device"
```

This is a raw device, Linux 4.5 and updated alsa 1.1.1. I get the same results with or without pulseaudio uninstalled. None of the 24-bit formats work and neither `device->formats` nor `soundio_device_supports_format` think it can do it either.

Should I contact the manufacturer or something? It's USB plug-and-play, so there are no drivers involved.