add support for neovim

[Cyperwu]

Notice that on vim 8.0+, job_status returns "run", "fail" or "dead". So I consider the job_status check branch is a bug.
Tested on nvim v0.5.0-6ca7ebb

[heavenshell]

@Cyperwu Thx for sending PR.
I'm currenlty bit busy now.
Maybe I can look into this weekend.

[heavenshell]

@Cyperwu
Can you add followings to .travis.yml?

  - env: ENV="Neovim"
    install:
      - mkdir -p ~/tmp/nvim/bin
      - curl -L https://github.com/neovim/neovim/releases/download/v0.4.3/nvim.appimage -o ~/tmp/nvim/bin/nvim
      - chmod u+x ~/tmp/nvim/bin/nvim

    script:
      - VIM_EXE=$HOME/tmp/nvim/bin/nvim ./run.sh

and than CI would be passed, I'll merge your PR.

[Cyperwu]

Sorry for late reply. I'll do it.

[heavenshell]

@Cyperwu Thank you for adding CI.
CI was failing.
https://travis-ci.org/github/heavenshell/vim-jsdoc/jobs/698386627#L265

Could you fix it?

[Cyperwu]

@heavenshell yep, still trying.

[vimark1]

@heavenshell @Cyperwu I created #92 to fix CI