Add option to disable inverting

[cozmo]

Inverting the image in the case of not finding a code in the initial image introduces a ~50% performance hit. This adds an options object to allow disabling the inversion if you don't expect to be scanning inverted QRs.

Defaults to the existing behavior (image inversion) but adds a note to the readme that this will probably be disabled in a future backwards incompatible change.

This also sets up the options object for #76.

[david-boles]

@cozmo What would you think about extending this new option to also support scanning only inverted and scanning inverted first? I think I'm going to be scanning images that mostly have inverted QR codes, so the later would make things a bit faster for me. It'd be more efficient for jsQR to do the inversion once it's binarized than me inverting all the RGB data before sending it to jsQR.

[cozmo]

Interesting,

What about something like attemptInverted: true | false | "first" | "only" - Where "first" would try the inverted one first, but still attempt both, and "only" would invert and then only try that one.

[david-boles]

That sounds good to me! Maybe make it an enum instead of boolean/string combo?

[alexandertrefz]

@cozmo
This seems like a major performance boost for those that don't need the inversion – can we cut a 1.1.2(since it defaults to the same behaviour) release from this?