Web.JSON: Add 'use_token' option to decode

If 'use_token' is specified in a option of decode function, the function
will translate javascript codes into corresponding special tokens

    let T = s:JSON.decode('true')                    " equal to 1
    let F = s:JSON.decode('false')                   " equal to 0
    let N = s:JSON.decode('null')                    " equal to 0

    let T = s:JSON.decode('true', {'use_token': 1})  " equal to s:JSON.true
    let F = s:JSON.decode('false', {'use_token': 1}) " equal to s:JSON.false
    let N = s:JSON.decode('null', {'use_token': 1})  " equal to s:JSON.null
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