Makes the Ghost application more express middleware friendly.

refs #827

- Moves ./index to use Ghost in a similar manner to how someone uses
Ghost as an npm module.
- Allows Ghost to be cleanly mounted on another express application
on any arbitrary endpoint, all you need to customize is the mount path.
Ability to create custom template files for custom tags. Eg. `tag-design.hbs`
Merge pull request #3902 from Chrisui/custom-tag-templates

Custom template files for custom tags
Merge pull request #3867 from hswolff/middleware-work

Makes the Ghost application more express middleware friendly.