Redirus should enable user to specify more configuration options for redirestions

Original problem was connected with openlab instances, which required sticky session:

```
upstream kibehmedx_http {
  ip_hash;
  server 10.100.20.11:80;
}
```

After looking at different solutions I propose to user [ERB](http://ruby-doc.org/stdlib-2.1.4/libdoc/erb/rdoc/ERB.html) for creating nginx configuration. It will have access to redirection name, ip addresses and properties. ERB also allows to embed ruby code, thus it should be easy to create quite complicated configuration.

TODOs:

 - [x] ERB template rendering
 - [x] update changelog
 - [x] add example how to create erb template into readme
 - [x] create init generator (`redirus init`) which will copy erb templates, simplest nginx configuration and template configuration.yml to current dir 
 - [ ] add possibility to pass properties using `redirus-cli`

/cc @djchrap
Use 2 spaces for indentation in an array, relative to the start of the line where the left bracket is.