Customisation of reference documentation using GFM

[zxchris]

Work in progress. The standard set of default pages need to be customisable. This is used extensively in the http://developer.companieshouse.gov.uk which uses the alpha implementation of Swaggerly.

The implementation will allow GFM pages to be created with custom anchors which reference identified locations in the theme page templates for API, method and resource, and Swaggerly will merge the GFM content into the generated reference page.

Allow content to be added to the default templates for:

1. An API (endpoint / path) summary page
2. An API method page
3. A resource page

additionally:

1. All method pages for a particular API endpoint

and

1. A specific API endpoint summary
2. A specific API endpoint method
3. A specific resource

[zxchris]

Complete.