Add a CharacterTable format that uses Unicode Control Pictures for whitespace and non-printable ASCII characters.

[PaulJuliusMartinez]

The Unicode Control Pictures are intended to be used as graphical representations of ASCII control characters (0x00 to 0x1f, and 0x7f), and space (0x20).

This pull request adds --character-table control-pictures to use these characters in the character table.

Unfortunately, font support for these is iffy, and some of the three-letter ones (NUL, DEL) appear as wide characters in terminals, so I'm not sure it would make sense for this to be the default. They are also quite small and can be difficult to read at small font sizes.

I also added tests the dump each character table as part of verifying a debug assertion that I added.

(Apologies for not opening an issue first; implementing it seemed simpler. I'm open to any and all changes.)

[sharkdp]

Thank you. Did you see https://github.com/sharkdp/hexyl/pull/247?