Now it's region based and uses keywords mode. Hopefully this is
faster. It also enables support for multiple libraries with partially
overlapping symbol sets. (E.g.: Boost and STL.)
This "cplusplus" filetype permits the sourcing of `cpp.vim`
and `doxygen.vim` as lower-level files from itself. This should
permit `doxygen.vim` to source from `cpp.vim` and avoid endless
recursion, when handling code-in-comments formatting.
