[RFC] CMake: Generate helptags during install step.

Fixes #1056 using the command from Vim's Makefile. It's not very portable yet; I'm sure there's a better way to express the following paths using some CMake variables:

> ${CMAKE_INSTALL_PREFIX}/bin/nvim
> "${CMAKE_INSTALL_PREFIX}/share/nvim/runtime/doc"

Ping @jszakmeister.