Convert Vim package management to actively managed package

[erictleung]

• Native package loading https://shapeshed.com/vim-packages/
• vim-plug package https://github.com/junegunn/vim-plug
• Use native loading: https://www.reddit.com/r/vim/comments/4qo7ur/which_plugin_installer_or_vim_package_manager/d4vikmj/

Related issue #11