Create a cmRST class to perform just enough reStructuredText processing
to support display of Help documents in human-readable text format.
This will be used to implement --help-* command-line options.
Support directives "include", "replace", "parsed-literal", "toctree"
(Sphinx), and "cmake-module" (CMake Sphinx Extension to scan .cmake
modules). Support inline CMake Sphinx Domain roles to convert
cross-references to corresponding title text. Support inline
substitutions defined by the "replace" directive, but keep it simple by
requiring replacements to be defined before use.
Add a CMakeLib "testRST" case to cover processing of supported
constructs and compare results against expected output.
Brad King