Changes to handling of different input file types

[Nigel Charman]

The markdown extension has revealed a few issues with the current extension mechanism for handling different input file types:

• Run command links have to end with .html (#3)
• Breadcrumbs expect a .html file extension (#4)
• If .md file not present fallback to html file (#7)

The mechanism currently relies on extensions to implement the following interfaces to add a new specification type:

• SpecificationLocator to find the input file
• Source to translate the input file
• Target to rename the output Resource to end with .html
  

I've created a PR that enables a fix for the above issues and makes it easier for extensions to add new specification types. Extensions will be able to call the single method ConcordionExtender.withSpecificationType(String typeSuffix, SpecificationConverter converter), with a simple converter that translates an input stream into an HTML format input stream.

I'm keen for feedback on this PR. This is a precursor to moving Markdown support into core (in addition to HTML format specs).

Nigel.