Allow fine-tuning for file compression

[cher-nov]

1. Allow compression levels in full ZLib 0-9 range. Currently only the next ones are supported:

 0 - ZStream.TCompressionLevel.clNone    (ZBase.Z_NO_COMPRESSION)
 1 - ZStream.TCompressionLevel.clFastest (ZBase.Z_BEST_SPEED)
 9 - ZStream.TCompressionLevel.clMax     (ZBase.Z_BEST_COMPRESSION)
-1 - ZStream.TCompressionLevel.clDefault (ZBase.Z_DEFAULT_COMPRESSION)

2. Allow specifying compression level per file.