Loading Homebrew Bash Completion if installed

In addition to the regular /etc/bash_completion, the one from Homebrew
is loaded when running on OS X, and when Homebrew is installed.

Also added the system completion to the default install options, since
this is a feature that's requested frequently.

install.sh

@@ -89,6 +89,7 @@ else
   echo ""
   echo -e "\033[0;32mEnabling sane defaults\033[0m"
   load_one completion bash-it.completion.bash
+  load_one completion system.completion.bash
   load_one plugins base.plugin.bash
   load_one plugins alias-completion.plugin.bash
   load_one aliases general.aliases.bash