Add support for varying endiannesses.

Currently only works with big and little endian. The base endianness -- that
is, the endianness of the output, is set by defining SZ_BASE_ENDIANNESS when
compiling snowball. The host endianness is set by defining SZ_ENDIANNESS.

If neither are specified, then the base and host endianness are both little
endian, because I haven't added a stage where the endianness is defined by a
configuration stage and there's no config file for snowball to load at the
moment.
Increment minor version number.

Magic version number remains the same since the defaults result in the same
output format by default.