Fix hard dependency on fzf.vim.
Fix hard dependency on fzf.vim.

Close #400.

autozimu/LanguageClient-neovim,autozimu/LanguageClient-neovim,autozimu/LanguageClient-neovim,autozimu/LanguageClient-neovim,autozimu/LanguageClient-neovim,autozimu/LanguageClient-neovim,autozimu/LanguageClient-neovim,autozimu/LanguageClient-neovim,autozimu/LanguageClient-neovim,autozimu/LanguageClient-neovim,autozimu/LanguageClient-neovim,autozimu/LanguageClient-neovim