SequenceFile should support 'custom compressors'

SequenceFiles should support 'custom compressors' which can be specified by the user on creation of the file.

Readily available packages for gzip and zip (java.util.zip) are among obvious choices to support. Of course there will be hooks so that other compressors can be added in future as long as there is a way to construct (input/output) streams on top of the compressor/decompressor.

The 'classname' of the 'custom compressor/decompressor' could be stored in the header of the SequenceFile which can then be used by SequenceFile.Reader to figure out the appropriate 'decompressor'. Thus I propose we add constructors to SequenceFile.Writer which take in the 'classname' of the compressor's input/output stream classes (e.g. DeflaterOutputStream/InflaterInputStream or GZIPOutputStream/GZIPInputStream), which acts as the hook for future compressors/decompressors.