Implement streaming preview window (#2215)

Fix #2212

    # Will start rendering after 200ms, update every 100ms
    fzf --preview 'for i in $(seq 100); do echo $i; sleep 0.01; done'

    # Should print "Loading .." message after 500ms
    fzf --preview 'sleep 1; for i in $(seq 100); do echo $i; sleep 0.01; done'

    # The first line should appear after 200ms
    fzf --preview 'date; sleep 2; date'

    # Should not render before enough lines for the scroll offset are ready
    rg --line-number --no-heading --color=always ^ |
      fzf --delimiter : --ansi --preview-window '+{2}-/2' \
          --preview 'sleep 1; bat --style=numbers --color=always --pager=never --highlight-line={2} {1}'

CHANGELOG.md

@@ -1,6 +1,13 @@
 CHANGELOG
 =========
 
+0.24.0
+------
+- fzf can render preview window before the command completes
+  ```sh
+  fzf --preview 'sleep 1; for i in $(seq 100); do echo $i; sleep 0.01; done'
+  ```
+
 0.23.1
 ------
 - Added `--preview-window` options for disabling flags