[RDY] 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, but I couldn't find it.~~ As the runtime is currently always installed into `share/nvim`, I guess this should be OK.

Ping @jszakmeister.