Added redis-cli interactive help support

updated via commands.json in redis-doc repo. Currently
use `make src/help.h` to re-generate. The following
are valid from the REPL:

  help
  help [command]
  help [group]
  help groups

ex:

  help sort
  help hash

utils/generate-command-help.rb

@@ -0,0 +1,56 @@
+#!/usr/bin/env ruby
+
+require 'net/http'
+require 'net/https'
+require 'json'
+require 'uri'
+
+dest = ARGV[0]
+tmpl = File.read './utils/help.h'
+
+url = URI.parse 'https://github.com/antirez/redis-doc/raw/master/commands.json'
+client = Net::HTTP.new url.host, url.port
+client.use_ssl = true
+res = client.get url.path
+
+def argument arg
+  name = arg['name'].is_a?(Array) ? arg['name'].join(' ') : arg['name']
+  name = arg['enum'].join '|' if 'enum' == arg['type']
+  name = arg['command'] + ' ' + name if arg['command']
+  if arg['multiple']
+    name = "(#{name})"
+    name += arg['optional'] ? '*' : '+'
+  elsif arg['optional']
+    name = "(#{name})?"
+  end
+  name
+end
+
+def arguments command
+  return '-' unless command['arguments']
+  command['arguments'].map do |arg|
+    argument arg
+  end.join ' '
+end
+
+case res
+when Net::HTTPSuccess
+  first = true
+  commands = JSON.parse(res.body)
+  c = commands.map do |key, command|
+    buf = if first
+      first = false
+      ' '
+    else
+      "\n  ,"
+    end
+    buf += " { \"#{key}\"\n" +
+    "  , \"#{arguments(command)}\"\n" +
+    "  , \"#{command['summary']}\"\n" +
+    "  , COMMAND_GROUP_#{command['group'].upcase}\n" +
+    "  , \"#{command['since']}\" }"
+  end.join("\n")
+  puts "\n// Auto-generated, do not edit.\n" + tmpl.sub('__COMMANDS__', c)
+else
+  res.error!
+end