feat(*): interactive deis run

This is a bit rough on the edges, but it works :bomb: 

```
Johannes:example-ruby-sinatra johannes$ deis run bash
Running `bash`...
ls
slug@954091b9a21b:~$ ls
bin  Gemfile  Gemfile.lock  LICENSE  Procfile  README.md  tmp  vendor  web.rb
echo "test"
slug@954091b9a21b:~$ echo "test"
test
env
slug@954091b9a21b:~$ env
HOSTNAME=954091b9a21b
TERM=xterm
RACK_ENV=production
DEIS_APP=truthy-chinbone
PATH=/app/bin:/app/vendor/bundle/bin:/app/vendor/bundle/ruby/1.9.1/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
PWD=/app
LANG=en_US.UTF-8
GIT_SHA=03a83a276110272005d707379c3acd282c79724d
SHLVL=2
HOME=/app
GEM_PATH=/app/vendor/bundle/ruby/1.9.1:
PORT=5000
DEIS_RELEASE=v2
_=/usr/bin/env
exit
slug@954091b9a21b:~$ exit
exit
0
```

Replaces https://github.com/deis/deis/pull/1837, https://github.com/deis/deis/pull/1875