#2 Support for extensions

[Pete Davis]

The library should support at least some of the common
extensions to NNTP as recognized in RFC 2980.

Among the more important are the XPAT, XHDR, XOVER,
XROVER, XTHREAD to allow faster retrieval of article
headers and threading information as well as the
AUTHINFO commands to allow for authentication which
most commercial servers require.

I have actually implemented XOVER in my copy. In doing
so, I added an IsPartial property to the Header class so
that the full header can be retrieve later as XOVER does
not retrieve complete header information. It is however,
suitable for getting a list of messages to display to the
user.

Support for LIST OVERVIEW.FMT could further be used
to allow for more extensive header retrieval where
supported.

Support for MODE READER could be used to improve
performance on the server side (where supported)