Add ability to create Redirect Domains.

This introduces an extra, optional, setting with a list of domains that should redirect to the main app.
  
## Usecases:
  * www to non-www, or viceversa redirection
  * Alternative domains that should redirect to the main app.
  
## Examples:
  ````@json
  domain_names: ['example.com']
  redirect_domain_names: ['www.example.com']
  ````
  www.example.com will now redirect to example.com
  ````@json
  domain_names: ['example.com', 'example.net']
  redirect_domain_names: ['example.org', 'www.example.org']
  ````
  example.org and www.example.org will now redirect to example.com. Application can be served on example.com and example.net.
  
It only handles redirects on domains. I cannot be used to redirect from/to https. And it cannot be used for rewrites or u    rl-redirects (like example.com/foo -> example.com/content/foo). Just domains.
  
  I'll add some documentation for the wiki in a followup comment.