Allow to compress payload with different compressor

Compressors are specified in description file.
For example:

[compressor]
gzip = *
none = *.gz, *.bz2
bzip2 = rootfs*

This description file set gzip as default compressor, do not compress payload
which end with .gz and .bz2 and use bzip2 as compressor for payload matching
'rootfs*'

Signed-off-by: Sébastien Luttringer <sebastien.luttringer@smartjog.com>