Support for intra-Markdown linking using [[WikiLinks]] syntax (issue-324)

Support for intra-Markdown linking using [[WikiLinks]] syntax (issue-324)

All WikiLinks must be specified relative to the root of the repository.
The displayed link text is stripped to just the document name. Spaces in
the document name are replaced with '-' characters; this is consistent with
wiki syntax and Github.

Change-Id: Id3fb1b5441352d9bacc4993a5b85882db113693b