Support customizing the location of ~/.pryrc

In Linux, there is a group that advocates less dotfiles in `$HOME` directly, instead moving all of those files into a dedicated `$HOME/.config` directory. This is the [XDG Base Directory Specification](http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html).

You don't have to go full-support of XDG, but I think changing [line 8 of `lib/pry/pry_class.rb`](https://github.com/pry/pry/blob/master/lib/pry/pry_class.rb#L8) to something along

```ruby
HOME_RC_FILE = ENV["PRYRC"] || "~/.pryrc"
```

Would be nice.