Support Private share.

[vdg0]

Add support here to handle private shares in a useful way.

[ConradIrwin]

At the moment no-one can access a showterm unless you give them the URL (like a private gist).

You can also run your own server if you're really paranoid.

Is there anything else you want?

[diwu1989]

Things I did to get a private showterm

• get rid of the Google Analytics code in the html
• increase the slug size to 32 characters

[ConradIrwin]

Sounds good. I'd happily merge that pull request.

Extending the length of the slug seems paranoid, but I didn't have a particularly good reason to choose the current length.