Add terminal target support for Vim 8

[munshkr]

From #26 (comment)

Perhaps it would also be beneficial to investigate the Vim8 terminal feature (See :help terminal).
I already tried using it in my own fork of this plugin (https://github.com/flupe/vim-tidal), it works quite well and gets rid of all the cumbersome tmux code and bash scripts altogether.
However maybe you are not willing to ditch the tmux part of this plugin, as it is possible some users do prefer to use tmux for this kind of things – I know I don't.

[flupe]

Hey, I have not kept my fork up to date but I'm glad this issue is being investigated. I'm willing to work on that if any help is needed. Just let me know.

[munshkr]

Hey! I will probably look into in the following weeks, but any help is appreciated, thanks!
It would be nice if we could extend these functions to include support for Vim 8. If Vim has a different API than NeoVim for the terminal, we'll probably have to add some conditionals e.g.:

if has("nvim")
  " nvim code
else
  " vim code
endif

[flupe]

I was stuck in a train for a couple hours so I decided to look at it, Still works fine on Vim8, haven't tested on Neovim nor tmux so I have yet to see if I broke anything.