Adds JSON output for replacements, to simplify tools integration.

Using -output-replacements will now output the replacements instead
of the changed code. This allows easier integration with tools that
need full control over what changed.

The format is an array of objects with the members "offset" (number),
"length" (number) and "replacement_text" (string), for example:

[
  {
    "offset": 42,
    "length": 5,
    "replacement_text": "  "
  },
  {
    "offset": 105,
    "length": 4,
    "replacement_text": ""
  }
]