New option for livestream

[davincho]

As discussed in #52 it is now possible to use any arbitrary element or selector, like so:

quagga.init({
  inputStream : {
    name : "Live",
    type : "LiveStream",
    target: document.querySelector("#myelement")
  }
})

or

quagga.init({
  inputStream : {
    name : "Live",
    type : "LiveStream",
    target: "#myelement"
  }
})

[serratus]

Instead of passing in the _config parameter, you could also use the global one (see L51)

[serratus]

Thanks for providing this PR. Works for me. Maybe you could drop one or two lines into the README to tell people how to use it (somewhere around https://github.com/serratus/quaggaJS#quaggainitconfig-callback).

[serratus]

Thanks for everything. Your changes will be made available in the next release which is scheduled in the next few days.