Allow users to specify both ref_in and ref_out when creating Digests.

[zachmse]

In the comments of #37, the ability to specify both ref_in and ref_out was discussed. I also think this capability would be good to add. If this change is made, I think it would be good to update the implementations of Digest to follow the builder pattern, rather than adding even more parameters to the new_custom() function. In fact, as the new_custom() function hasn't been included in a release yet (as far as I can tell), it could even be removed.

For example, I envision something like the following:

// Anything not explicitly set is given a "sane" default value
let mut digest = crc16::Digest::new(poly)
    .initial_crc(crc)
    .reflection_in(true)
    .reflection_out(true);

Ultimately I don't have a strong preference on how these values are set, so feel free to "no change" this issue, but I would be more than happy to prototype the changes if this is something that might be wanted.

[mrhooray]

closing as #55 has been merged