Support backticks in property values

[cyberbiont]

Currently jsonic converts `string` to "`string`"

[rjrodger]

some example code would actually be useful here - thanks

[cyberbiont]

Take the following JS:

{
    a: `b`,
}

copy it to JSON file

Run it through Jsonic
you'll get

{
  "a": "`b`"
}

instead of

{
  "a": "b"
}

Backticks should be stripped out