JSON report

I would like the diffenator to produce a JSON (or other machine readable) report, of the form:

```
{
new_encoded_codepoints: [ 0xA123, ... ],
missing_encoded_codepoints: [ 0xA124, ... ],
modified_encoded_glyphs: [ 'glyphname', ... ],
total_strings_tested: 12345,
misshapen_strings: [ ... ]
}
```

This is so that I can programmatically generate a report which says something like "You should upgrade font A because it encodes the additional characters 0xA123, ..." and "You do not need to upgrade font B because there are no encoding changes, no glyph shaping changes, and we tested 12345 strings and there were no regressions".