[preview] Clean up preview scripts

- Use preview.sh instead of preview.rb by default
- Try bat syntax highlighter with the highest priority
- Remove "-v" option as the latest version of fzf sets up $LINES
- Allow users to customize the preview command via $FZF_PREVIEW_COMMAND
  - *EXPERIMENTAL / EVOLVING / UNDOCUMENTED*
  - Should be a command template with {} placeholder expression
    - e.g. "nl {}"

bin/preview.rb

@@ -1,18 +1,20 @@
 #!/usr/bin/env ruby
 #
-# usage: ./preview.rb [-v] FILENAME[:LINE][:IGNORED]
+# usage: ./preview.rb FILENAME[:LINE][:IGNORED]
 
+require 'open3'
 require 'shellwords'
 
-COMMAND = %[(highlight -O ansi -l {} || coderay {} || rougify {} || bat --style=numbers --color=always {} || cat {}) 2> /dev/null]
+COMMAND = ENV.fetch(
+  'FZF_PREVIEW_COMMAND',
+  %[bat --style=numbers --color=always {} || highlight -O ansi -l {} || coderay {} || rougify {} || cat {}]
+)
 ANSI    = /\x1b\[[0-9;]*m/
 REVERSE = "\x1b[7m"
 RESET   = "\x1b[m"
 
-split = ARGV.delete('-v')
-
 def usage
-  puts "usage: #$0 [-v] FILENAME[:LINENO][:IGNORED]"
+  puts "usage: #$0 FILENAME[:LINENO][:IGNORED]"
   exit 1
 end
 
@@ -37,17 +39,16 @@ if `file --mime "#{file}"` =~ /binary/
 end
 
 center = (center || 0).to_i
-if ENV['LINES']
-  height = ENV['LINES'].to_i
-else
-  height = File.readable?('/dev/tty') ? `stty size < /dev/tty`.split.first.to_i : 40
-  height /= 2 if split
-  height -= 2 # preview border
-end
+height =
+  if ENV['LINES']
+    ENV['LINES'].to_i
+  else
+    File.readable?('/dev/tty') ? `stty size < /dev/tty`.split.first.to_i : 40
+  end
 offset = [1, center - height / 3].max
 
-IO.popen(['sh', '-c', COMMAND.gsub('{}', Shellwords.shellescape(path))]) do |io|
-  io.each_line.drop(offset - 1).take(height).each_with_index do |line, lno|
+Open3.popen3(COMMAND.gsub('{}', Shellwords.shellescape(path))) do |_in, out, _err|
+  out.each_line.drop(offset - 1).take(height).each_with_index do |line, lno|
     if lno + offset == center
       puts REVERSE + line.chomp.gsub(ANSI) { |m| m + REVERSE } + RESET
     else