GeezerVison

Fixes #362.

--palette values were copied as is, so --palette='ad=\061;32' put a literal \061 in the output, and \: ended the value.

Values are now decoded the way GNU diff 3.12 does it:

• \NNN octal and \xHH hex (any number of digits, low byte kept), \a \b \e \f \n \r \t \v, \? for DEL, \_ for space; any other escaped character stands for itself, so \: and \\ work
• ^X caret notation for @ to ~, and ^? for DEL
• a value ending in \ or ^, or ^ followed by something else, is rejected with the same two messages as GNU (unrecognized prefix: ad, unparsable value for --palette)

The palette entries are bytes now, since a value can decode to anything.

One difference left on purpose: for ^? GNU prints DEL followed by a stray ?, which looks like a bug there. This prints DEL only.