Support for multiple hostnames

[nottrobin]

We need the ability for this application to understand and serve several domains. E.g.:

• docs.vanillaframework.io
• docs.snapcraft.io
  etc.

So the app needs updating to read a config file which will map hostnames to set of documentation. E.g.:

docs.vanillaframework.io: templates/vanillaframework/
docs.snapcraft.io: templates/snapcraft/

It should also then redirect from docs.ubuntu.com/{product} to docs.{product}.io in these cases.

This is important specifically for Vanillaframework and Snapcraft, because neither can host their documentation on this platform, as they should, until this is implemented.

[yaili]

@anthonydillon @nottrobin do we need to prioritise this issue?

[nottrobin]

Yes we should. I'll ask @frankiegranato to add it to the web squad backlog.

[nottrobin]

This will be fixed through #86