Replace unite with denite

[kiryph]

According to https://github.com/Shougo/unite.vim/blob/master/README.md

Note: Active development on unite.vim has stopped. The only future changes will be bug fixes.

Please see Denite.nvim.

However, denite requires neovim or vim8. (maybe one can still mention unite for vim7 users)

[mhinz]

Thanks!