digraphs: alternate glyph string in include
Allow the digraph include to specify string overrides in cases where a
glyph should not be shown literally. These are:
- Combining characters: prepend a placeholder.
Dead chars are invalid on their own in fact.
- ASCII control characters: substitude display symbols at U+24xx.
Though browsers usually show a character placeholder, it's not very
nice to send control chars directly.
- Other control characters: show the replacement character U+FFFD.
Actually with some (Linux) fonts, the anonymous code point fallback is
more descriptive, but better to be on the safe side (they still have
semantic value after all).