Add support for RAR v5.0

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Currently RAR5 is not supported by SevenZipSharp, I had a look at #11 and thought it might be a quick thing to add, but the problem is that both RAR4 and RAR5 use the .rar extension, so to add RAR5 it seems you'd have to overwrite RAR4 (and I can't find any mention on whether or not you can open a RAR4 file the same way as a RAR5 file).

In theory what's needed is to change /SevenZip/Formats.cs:

• Add/change the RAR5 GUID 23170f69-40c1-278a-1000-000110CC0000.
• Add/change its signature: 52-61-72-21-1A-07-01-00.

But the file extension confuses me, you can't add two duplicate extensions to InExtensionFormats.