Ability to write to custom writer and / or std.log

[JanBeelte]

I think it would be of great value if a writer could be supplied to the dump method ala:

try spew.dump(m, .{writer = my_writer});

also sane interaction with zigs logger would be awesome e.g.:

try spew.dump(m, .{logger=.info}); // writing to std.log.info