[RFC] CMake: Generate helptags during install step.

Fixes #1056 using the [command from Vim's Makefile](https://code.google.com/p/vim/source/browse/runtime/doc/Makefile#312). It's not very portable yet; I'm sure there's a Windows-compliant way to express `${CMAKE_INSTALL_PREFIX}/share/nvim/runtime/doc` using some CMake variable.

Ping @jszakmeister.
Can't use `${CMAKE_INSTALL_PREFIX}/bin/nvim` here because the `nvim` install target is in a subdirectory and will thus run after this one.