add linting, tests and CI

[prabirshrestha]

Ideally I would like to have all the tools be pure vimscript so it is easy for anyone to contribute without installing extra tools. Getting tools such as python is not easy for windows dev.

For CI I would like to have support for multiple OS. While I'm ok with starting with Linux first, Windows is equally important since that is my work machine and Mac is my home and OSS machine.

Feel free to recommend tools in comments or go ahead and write a simple small test and send a PR so we can evaluate or point to other vim plugin tests/lint/ci setups.

• [x] Linting via vlint
• [x] Linux CI support
• [x] Mac CI support
• [x] Windows CI support
• [ ] run specific version of vim 8
• [ ] run tests on master vim8
• [x] run on specific version of neovim
• [x] run on nightly neovim
• [ ] reviewdog support for vlint

[prabirshrestha]

Themis test is now setup with travis to run agains neovim and vim.