Perl: Add word of caution about keeping constants in sync
Perl: Fixes for compression related options

a) It is "compress", not "compression".
b) Only bother looking at certain compression related
   options if compression is even enabled.
c) Do not bother with the old Snappy specific options
   if Snappy is not enabled.
Perl: Document new compression options
Hobodecoder: Incremental Snappy support
Perl: Comment nits (cosmetics)
Hobodecoder: LZ4 support