Add g:fzf_grep_preview_window for enabling Ag/grep preview

Close #225

bin/preview.rb

@@ -0,0 +1,26 @@
+#!/usr/bin/env ruby
+#
+# usage: ./preview.rb FILENAME LINENO VSPLIT(0|1)
+
+require 'shellwords'
+
+COMMAND = %[(highlight -O ansi -l {} || coderay {} || cat {}) 2> /dev/null]
+ANSI    = /\x1b\[[0-9;]*m/
+REVERSE = "\x1b[7m"
+RESET   = "\x1b[m"
+
+file, center, split = ARGV.fetch(0), ARGV.fetch(1).to_i, ARGV.fetch(2) == '1'
+height = File.readable?('/dev/tty') ? `stty size < /dev/tty`.split.first.to_i : 40
+height /= 2 if split
+height -= 2 # preview border
+offset = [1, center - height / 3].max
+
+IO.popen(['sh', '-c', COMMAND.gsub('{}', Shellwords.shellescape(file))]) do |io|
+  io.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
+      puts line
+    end
+  end
+end