Temporarily add a RST formatter to convert builtin documentation to .rst
source files. This will be removed shortly after we use it to convert
documentation.
Teach the RST formatter to:
* Output preformatted blocks as reStructuredText "::" literal blocks.
* Output option lists as bullet lists with option names enclosed in
reStructuredText ``literal`` quoting.
* Output individual documentation objects (commands, variables, etc.)
in separate .rst files organized in directories by type.
Replace references to cmVersion::GetCMakeVersion() in builtin
documentation with the literal placeholder "|release|" that will be
defined as a substitution later.
Brad King