Add Extra Dependencies To Support SNI

Then we could install httpie in [pipsi](https://github.com/mitsuhiko/pipsi) with SNI support:

```
$ pipsi install httpie[security]
```

It is the same as [requests](https://github.com/kennethreitz/requests/blob/359659cf4b9dbeeef1ed832501dc1f99b0f0beac/setup.py#L65-L67) done.

Related Topics:

- https://github.com/kennethreitz/requests/issues/749
- https://github.com/jakubroztocil/httpie/issues/262
- https://github.com/jakubroztocil/httpie/issues/154