Minor enhancements

This PR adds the following changes:

* A new command `ohmygems env` to print the environment information. I found I prefered this to having it printed each time ohmygems was called.
* Store gem environment name in `OMG_NAME` environment variable. I wanted to have an easy way of knowing if I was in a gem environment so I could display it in my prompt.
* Create the gem environment directory when switching to a name that doesn't already exist. I usually create a separate gem environment for each project I work on, and this saved me from printing the usage to see where the directory should be.