Add flag json-reformat:pretty-string?

[gongo]

Motivation

see GH-12

Example

{"name":"foobar","nick":"foo \u00e4 bar","description":"<pre>\nbaz\n</pre>"}

If (setq json-reformat:pretty-string? nil):

    {
        "name": "foobar",
        "nick": "foo \u00e4 bar",
        "description": "<pre>\nbaz\n<\/pre>"
    }

Else (setq json-reformat:pretty-string? t):

    {
        "name": "foobar",
        "nick": "foo ä bar",
        "description": "<pre>
    baz
    </pre>"
    }

[coveralls]

Coverage remained the same when pulling d4eea08 on add_option_pretty_string into a99387e on master.

[coveralls]

Coverage remained the same when pulling d4eea08 on add_option_pretty_string into a99387e on master.

[coveralls]

Coverage remained the same when pulling 973366f on add_option_pretty_string into a99387e on master.

[coveralls]

Coverage remained the same when pulling 7e61723 on add_option_pretty_string into a99387e on master.