Add PayPal donation button

Close #893

Thanks to @majkinetor. Close #869.

CTRL-R binding used to start with --no-sort to list the matched commands
in chronological order. However, it has been a constant source of
confusion. Let's enable it by default from now on. The sorted result
shouldn't be too confusing as we use --tiebreak=index.

Goes well with --height

Replace the "temp file" workaround with the "read" function: it's
simpler and faster.

Use proper escaping, remove the custom function.

The "file" widget uses last token as root for the "find" command.
This replaces the equivalent of '**' completion in bash/zsh.
The "$dir" non-expanded variable can be used in FZF_CTRL_T_COMMAND to
set the root.

Close #726

Close #711

fzf#wrap([name string,] [opts dict,] [fullscreen boolean])

Close #627

Close #596

Close #560, #561

Notes:
- You can now override _fzf_compgen_path and _fzf_compgen_dir functions
  to use custom commands such as ag instead of find for listing
  completion candidates.
    - The first argument is the base path to start traversal
- Removed file-only completion in bash, i.e. _fzf_file_completion.
  Maintaining a list of commands that only expect files, not
  directories, is cumbersome (there are too many) and error-prone.

TBD:
- Added $FZF_COMPLETION_DIR_COMMANDS to customize the list of commands
  which use directory-only completion. The default is "cd pushd rmdir".
  Not sure if it's the best approach to address the requirement, I'll
  leave it as an undocumented feature.

Related: #406 (@thomcom), #456 (@frizinak)

Close #412

Close #400

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

Close #338