Close https://github.com/junegunn/fzf.vim/issues/308

- call fzf#run({'source': "echo '!'"})
- call fzf#run({'source': "echo '!'", 'down': '40%'})

Close https://github.com/junegunn/fzf.vim/issues/315

Close #847. Patch submitted by @wontoncc.

Close https://github.com/junegunn/fzf.vim/issues/309

Fix broken preview border. Reported by Thomas Sattler.

    fzf --bind 'enter:execute(date)' --preview=date --reverse

Using a dummy command that works as the barrier.

Due to the recent removal of ncurses dependency, we can cross-compile
binaries for different platforms without virtual machines.

Related #832

To override --height option in FZF_DEFAULT_OPTS

Goes well with --height

Due to performance consideration, FuzzyMatchV2 does not return the exact
positions of the matching characters by default. However, the ommission
caused `--tiebreak=begin` to produce inaccurate result in some cases.

  (echo baz foo bar; echo foo bar baz) | fzf --tiebreak=begin -fbar | head -1

  # Expected: foo bar baz
  # Actual:   baz foo bar

This commit fixes the problem by using the end offset which is
guaranteed to be correct.

https://github.com/junegunn/go-runewidth/pull/1

/cc @joshuarubin

Exposing the `b:fzf` variable will be useful to get information about
which command is being executed on the current fzf window. With that,
now, it's possible to use the current command name on the statusline:

```viml
au User FzfStatusLine call <SID>fzf_statusline()

function! s:fzf_statusline()
  let fzf_cmd_name = get(b:fzf, 'name', 'FZF')
  let &l:statusline = '> '.fzf_cmd_name
endfunction
```

https://github.com/junegunn/fzf.vim/issues/296

- Move cursor to the top-left corner when starting a command in
  alternate screen
- Fix cursor position when returning to alternate screen when fzf is
  running in full screen mode

Close #825

Close #823

Close #788