Add support for subscribers beta

[ErisDS]

Hi there!

Ghostbot currently has no support for subscribers beta. This is usually OK as Ghost has enough default behaviour baked in that themes don't explicitly need to add support.

However, the fancy ajax navigation which is the main feature of ghostbot breaks with the subscribers template, because the default template doesn't have #ajax-container in the HTML.

It's quite usual for people enabling the feature to add a /subscribe/ URL to their navigation section in Ghost, and with this particular theme, that won't work. The link will always go to an empty page, until you refresh and the /subscribe/ page loads. This is quite confusing behaviour.

Whilst adding this, it would be great to also add a conditional form to the post.hbs :) Documentation is here http://themes.ghost.org/docs/subscribers

Cheers ☮️