Add support for converting text to Json

[nickbabcock]

The mid-level API now provides the excellent utility of converting the
plaintext Clausewitz format to JSON when the json feature is enabled.

use jomini::TextTape;

let tape = TextTape::from_slice(b"foo=bar")?;
let reader = tape.windows1252_reader();
let actual = reader.json().to_string()?;
assert_eq!(actual, r#"{"foo":"bar"}"#);

The scope of the JSON can be narrowed to an inner value. This comes in handy
when the parsed document is large but only a small subset of it needs to be
exposed with JSON.

use jomini::{TextTape};

let tape = TextTape::from_slice(b"nums={1 2 3 4}")?;
let reader = tape.windows1252_reader();
let mut fields = reader.fields();
let (_key, _op, value) = fields.next().unwrap();
let array = value.read_array()?;
let actual = array.json().to_string()?;
let expected = r#"[1,2,3,4]"#;
assert_eq!(&actual, expected);

It's debatable whether duplicate keys is valid JSON, so this allows one
to customize output depending how flexible a downstream client is at handling
JSON.

The options are either:

• Group values into an array under a single field
• Preserve the duplicate keys
• Rewrite objects as an array of key value pairs