Omz command line tool

I wrote a simple command line tool that dinamically loads plugins.

Example: type
```omz plugin rvm```

to source all commands/autocompletion for rvm into current session. This can be handy when you don't want to slow down zsh boot time with plugins you use very few times, or when you want to try a new one.

Command omz can also manage other oh my zsh tools, like theme_chooser.sh or upgrade.sh, or anything that will be added in the tools/ folder. There's also an autocompletion file for it.

__TODO__: currently I can't find a clean way to unload plugins. A solution could be sourcing a special file like _plugin/rvm/rvm.disable.zsh_ with all the necessary instructions to remove the plugin.