Introduced new parameter CompressionMode for Compress-7Zip cmdlet.

[itmagination]

I would like to introduce new parameter CompressionMode for Compress-7Zip cmdlet. It allows expanding existing archives by passing it Append value. Underneath it is just an exposition of SevenZipCompressor's CompressionMode option. By default its value is of course Create, as it was previously.

[thoemmi]

Thanks for your contribution. However, instead of adding a parameter CompressionMode, I added a switch Append (see 3af1d65). I think it's easier for users to understand.

It's included in v1.7, which was released today.