Add command-line flags to install script

Close #392

  usage: ./install [OPTIONS]

      --help               Show this message
      --bin                Download fzf binary only
      --all                Download fzf binary and update configuration files
                           to enable key bindings and fuzzy completion
      --[no-]key-bindings  Enable/disable key bindings (CTRL-T, CTRL-R, ALT-C)
      --[no-]completion    Enable/disable fuzzy completion (bash & zsh)
      --[no-]update-rc     Whether or not to update shell configuration files

README.md

@@ -68,7 +68,7 @@ Or you can have [vim-plug](https://github.com/junegunn/vim-plug) manage fzf
 (recommended):
 
 ```vim
-Plug 'junegunn/fzf', { 'dir': '~/.fzf', 'do': 'yes \| ./install' }
+Plug 'junegunn/fzf', { 'dir': '~/.fzf', 'do': './install --all' }
 ```
 
 #### Upgrading fzf