Add an option so that only the admin can post.

[sebsauvage]

I received two requests for this feature, and I think it can be useful.

• Only the admin can post a paste (using a password)
• Anybody can read (without password)

In user mode, the new/clone buttons would be disabled.

[Ypnose]

Hi, any updates?

[ulikoehler]

I suppose this is easier to implement after #63 has been merged -- you could just create a config option then.