Added SupportsRandomAccess property to IArchiveFactory

[vpenades]

I've been using tgz files and I've realized they require being opened using the IReader interface to read them sequentially.

I thought it could be useful to embed that knowledge in the IArchiveFactory.

So both TAR and GZip factories have that property set to false.

This way, a program that wants to open any kind of archive, would know beforehand which is the preferred way of reading the archive and adjust accordingly.

Also added the "tgz" extension to tar factory.... I guess it would also require "tar.gz" but I'm hesitant to add extensions with extra dots.

[vpenades]

Hmm... I think there's a better solution for this... closing!

[adamhathcock]

I like the idea of trying to tell people when using the API. Keep going!

[vpenades]

opened an issue #708 to discuss before opening another PR