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

utils/help.h

@@ -0,0 +1,119 @@
+
+#include <stdio.h>
+#include <string.h>
+
+/*
+ * List command groups.
+ */
+
+#define GROUPS \
+  G(UNKNOWN, "unknown") \
+  G(SET, "set") \
+  G(LIST, "list") \
+  G(HASH, "hash") \
+  G(GENERIC, "generic") \
+  G(PUBSUB, "pubsub") \
+  G(STRING, "string") \
+  G(SERVER, "server") \
+  G(CONNECTION, "connection") \
+  G(TRANSACTIONS, "transactions") \
+  G(SORTED_SET, "sorted_set")
+
+/*
+ * Command group types.
+ */
+
+typedef enum {
+  #define G(GROUP, _) COMMAND_GROUP_##GROUP,
+  GROUPS
+  #undef G
+  COMMAND_GROUP_LENGTH
+} command_group_type_t;
+
+/*
+ * Command group type names.
+ */
+
+static char *command_group_type_names[] = {
+  #define G(_, STR) STR,
+  GROUPS
+  #undef G
+};
+
+/*
+ * Command help struct.
+ */
+
+struct command_help {
+  char *name;
+  char *params;
+  char *summary;
+  command_group_type_t group;
+  char *since;
+} command_help[] = {
+  __COMMANDS__
+};
+
+/*
+ * Output command help to stdout.
+ */
+
+static void
+output_command_help(struct command_help *help) {
+  printf("\n  \x1b[1m%s\x1b[0m \x1b[90m%s\x1b[0m\n", help->name, help->params);
+  printf("  \x1b[33msummary:\x1b[0m %s\n", help->summary);
+  printf("  \x1b[33msince:\x1b[0m %s\n", help->since);
+  printf("  \x1b[33mgroup:\x1b[0m %s\n", command_group_type_names[help->group]);
+}
+
+/*
+ * Return command group type by name string.
+ */
+
+static command_group_type_t
+command_group_type_by_name(const char *name) {
+  for (int i = 0; i < COMMAND_GROUP_LENGTH; ++i) {
+    const char *group = command_group_type_names[i];
+    if (0 == strcasecmp(name, group)) return i;
+  }
+  return 0;
+}
+
+/*
+ * Output group names.
+ */
+
+static void
+output_group_help() {
+	for (int i = 0; i < COMMAND_GROUP_LENGTH; ++i) {
+		if (COMMAND_GROUP_UNKNOWN == i) continue;
+		const char *group = command_group_type_names[i];
+		printf("  \x1b[90m-\x1b[0m %s\n", group);
+	}
+}
+
+/*
+ * Output all command help, filtering by group or command name.
+ */
+
+static void
+output_help(int argc, const char **argv) {
+  int len = sizeof(command_help) / sizeof(struct command_help);
+
+	if (argc && 0 == strcasecmp("groups", argv[0])) {
+		output_group_help();
+		return;
+	}
+
+	command_group_type_t group = argc
+    ? command_group_type_by_name(argv[0])
+    : COMMAND_GROUP_UNKNOWN;
+
+  for (int i = 0; i < len; ++i) {
+    struct command_help help = command_help[i];
+    if (argc && !group && 0 != strcasecmp(help.name, argv[0])) continue;
+    if (group && group != help.group) continue;
+    output_command_help(&help);
+  }
+  puts("");
+}