[add] github actions

[get-me-power]

Hello, @lambdalisue .

It same Pull Request #241

I replaced unit test from .travis.yml. What do you think this patch? If you don't need this pull request, feel free to close it. 🙇

reference

https://github.com/kazukazuinaina/gina.vim/pull/1/checks?check_run_id=433354167

ps: ci falled but it may not be problem because ci has failed simply because the fork destination and root url are different

replaced

• vint check
• Vim's unit test
• Neovim's unit test

unreplaced

• codecov

[lambdalisue]

Well, I'm sorry but I'm planning to replace Github Action to run all tests for all platform thus I'll do it myself.