Support for different code pages / character tables

[sharkdp]

I found this cool blog post by @delan today and I think we should experiment with adding support for different code pages to hexyl. We currently print different categories of bytes using ⋄, × or •, but it would be nice to be able to distinguish each byte. Maybe we could add a --character-table command line option (or similar) that could be used to switch to different byte => Unicode code point mappings. The default would (probably?) be what we have right now, but we could then set e.g. --character-table codepage-437 to use Code page 437.