Close #1379

- Update preview window even if there is no match for the query string
  if any of the placeholder expressions evaluates to a non-empty string.
- Also, if the command template contains {q}, preview window will be
  updated if the query string changes even though the focus remains on
  the same item.

An example:

    git log --oneline --color=always |
       fzf --reverse --ansi --preview \
       '[ -n {1} ] && git show --color=always {1} || git show --color=always {q}'

Close #1307

Close #1234

For now, they are respectively bound to preview-up and preview-down
by default (TBD).

Not available on tcell build.

Close #1201

This reverts commit 2f1edeff.

'accept-non-empty' is similar to 'accept' (which is bound to 'enter' and
'double-click' by default) but it prevents fzf from exiting without any
selection.

Close #1162

replace-query action replaces the query string with the current
selection. If the selection is too long, it will be truncated.

If the line contains meta-characters of fzf search syntax, it is
possible that the line is no longer included in the updated result.

e.g.

  echo '!hello' | fzf --bind ctrl-v:replace-query

Close #1137

Close #1168

left-click and right-click are respectively bound to "ignore" and
"toggle" (after implicitly moving the cursor) by default.

Close #1130

Similarly to --bind or --color.

--expect used to replace the previously specified keys, and
fzf#wrap({'options': '--expect=f1'}) wouldn't work as expected. It
forced us to come up with some ugly hacks like the following:

https://github.com/junegunn/fzf.vim/blob/13b27c45c8bdf6c3a41376bb83e4895edadf8c7e/autoload/fzf/vim.vim#L1086

Close #1003

    # Move cursor to the top result whenever the query string is changed
    fzf --bind change:top

Close #925

Close #906

Close #888
Close #894

Goes well with --height

Close #825

Close #823

Close #788