Restore the original line when search is aborted. Add --query
"$READLINE_LINE" and fall back to the current behavior pre Bash 4.

Co-authored-by: Junegunn Choi <junegunn.c@gmail.com>

- Fix 'make docker-test'
- Set fish_history to an empty string since 'fish --private' is not
  available prior to fish 3.0

'backward-delete-char/eof' will either abort if query is
empty or delete one character backwards.

Close #1744

Close #1370 

Parses the history list, converts it to a NUL-delimited list of possibly
multiline entries. Adds the fzf --read0 option. Works with and without
histexpand enabled.

Co-authored-by: Junegunn Choi <junegunn.c@gmail.com>

Make C-t more consistent pre and post Bash 4. It already kills the
command line separately before and after the insertion point. Add
set-mark and exchange-point-and-mark to restore the insertion point
after yanking back and apply the same behavior to M-c.

* CTRL-T should put extra space after pasted items

Co-authored-by: Junegunn Choi <junegunn.c@gmail.com>

Be consistent with emacs mode and put the items at the point vs. the end
of the command line.

Related #1809 #1850

Fix #1877

Add --pointer and --marker option which can provide additional context to the user

When jump-labels are specified with `--jump-labels=` way, validation was
not carried out.

Close #1874

e.g.

  let g:fzf_layout = { 'window': { 'width': 0.4, 'height': 1, 'xoffset': 0, 'border': 'right' } }
  let g:fzf_layout = { 'window': { 'width': 0.4, 'height': 1, 'xoffset': 1, 'border': 'left' } }
  let g:fzf_layout = { 'window': { 'width': 1, 'height': 0.5, 'yoffset': 1, 'border': 'top' } }
  let g:fzf_layout = { 'window': { 'width': 1, 'height': 0.5, 'yoffset': 0, 'border': 'bottom' } }

Revert the change introduced in #552. It seems that the startup time
difference between bash and fish is not much of an issue now.

  > time bash -c 'date'
  Thu Feb 13 21:15:03 KST 2020

  real    0m0.008s
  user    0m0.003s
  sys     0m0.003s

  > time fish -c 'date'
  Thu Feb 13 21:15:05 KST 2020

  real    0m0.014s
  user    0m0.007s
  sys     0m0.006s

When we explicitly *pipe* $FZF_DEFAULT_COMMAND instead of making fzf
internally start the process ($FZF_DEFAULT_COMMAND | fzf), fzf may hang
if the input process doesn't quickly process SIGPIPE and abort.

Also, fzf#vim#grep temporarily swaps $FZF_DEFAULT_COMMAND instead of
setting 'sink' so fzf can kill the default command on 'reload'.

https://github.com/junegunn/fzf.vim/issues/927

However, because of the "pipe conversion", the trick wasn't working as
expected.

https://github.com/junegunn/fzf.vim/blob/467c3277884240f7b5430f8f4d600e3415c38f3b/autoload/fzf/vim.vim#L720-L726

We can go even further and always set $FZF_DEFAULT_COMMAND instead of
piping source command.

Credits to @jiangjianshan

Close #1856
Close #1857

Close https://github.com/junegunn/fzf.vim/issues/942

Makes scanning 300x faster on Windows

e.g.
  let g:fzf_layout = { 'window': { 'width': 0.9, 'height': 0.6 } }

Based on the code from https://github.com/junegunn/fzf.vim/issues/821#issuecomment-581273191
by @lacygoill.

Update copyright year to 2020 and change to multi-year format.

Fix #1637