Custom (de)compression via --{,de}compcmd options

[woky]

This came from need to provide additional decryption parameters to GPG,
which is currently not possible. Instead of adding --gpg-decrypt-extra
option, this commit lets users provide custom commands (de)compression.

Example usage:

./makeself --compcmd "gpg -c"
--decompcmd "gpg -d --pinentry-mode=loopback"
. /tmp/sfx.sh installer ./run

[megastep]

That sounds good. Would you mind adding some unit tests for this?

[woky]

Sure, I'll poke it over the weekend.

There are lots of temp=`mktemp -d -t XXXXX` in tests but I don't see any rm -r $temp on exit. Are temporary directories cleaned up in some other way or is it intentional to keep them? If neither, then TMPDIR could be set to yet another mktemp -d in Makefile and rm -red on success.