Use '--with-mecab' instead of '--use-homebrew-mecab'

Homebrew #26308 was merged but it was changed to meet Homebrew's style.
So, I've changed formula to follow it.

mroonga.rb

@@ -10,8 +10,8 @@ class Mroonga < Formula
   depends_on 'pkg-config' => :build
   depends_on 'groonga-normalizer-mysql'
 
-  if ARGV.include?("--use-homebrew-mecab")
-    depends_on 'groonga' => '--use-homebrew-mecab'
+  if build.with?("mecab")
+    depends_on 'groonga' => '--with-mecab'
   else
     depends_on 'groonga'
   end
@@ -26,7 +26,7 @@ def options
     [
       ["--use-homebrew-mysql", "Use MySQL installed by Homebrew."],
       ["--use-homebrew-mariadb", "Use MariaDB installed by Homebrew. You can't use this option with --use-homebrew-mysql."],
-      ["--use-homebrew-mecab", "Use MeCab installed by Homebrew. You can use additional tokenizer - TokenMecab."],
+      ["--with-mecab", "Use MeCab installed by Homebrew. You can use additional tokenizer - TokenMecab. Note that you need to build Groonga with MeCab"],
       ["--with-mysql-source=PATH", "MySQL source directory. You can't use this option with --use-homebrew-mysql and --use-homebrew-mariadb"],
       ["--with-mysql-build=PATH", "MySQL build directory (default: guess from --with-mysql-source)"],
       ["--with-mysql-config=PATH", "mysql_config path (default: guess from --with-mysql-source)"],