Introduce the option to hide special buffers

Some "boring" buffers can now optionally be hidden by
the users via the use of the `nrepl-hide-special-buffers` setting.
By default all special buffers are visible.
Fix nrepl-hide-special-buffers docstring.
Fix test names.
Merge pull request #255 from bbatsov/hide-bufffers

Introduce the option to hide special buffers