CMakeLists: add install target for nvim
Makefile: add install target

This simply calls the install target in the build directory. IMHO I
think it's looking a bit hacky having a separate Makefile target to do
this rather than using the usual CMake workflow but mine is not to
reason why... [Also, I've copied ``cd build && make ...`` although I'm
sure ``$MAKE -C build/ ...`` is probably the Right Thing (TM).]

Note that you'll have to set CMAKE_INSTALL_PREFIX on the cmake command
line to change where this installs to.
Makefile: allow configuration of CMake flags

Although CMAKE_FLAGS was already a Makefile variable, it didn't have an
empty default value meaning that extending the flags to CMake in a clean
way was difficult. Add a CMAKE_EXTRA_FLAGS variable which is appended to
the default flags.
add installation as a travis test

Not only should we pass the test suite but we should also install
successfully.