Adds constants to specify index file generation. Refs #34.
Implements generation of index file for tools. Refs #34.
Adds idx file generation to test suite. Refs #34.
Adds document title to generated index toc. Refs #34.

This conforms to the title index specification documented at
http://nim-lang.org/docgen.html#index-idx-file-format.
Documents task in changes log. Refs #34.
Merge branch 'feature/t34_generate_idx_files'