Separate out blink(1) commands into blink1.js.

Create a separate Javascript class that represents a particular blink(1)
device and encapsulates the usage of the chrome.hid API.
Because `getRgb` can return undefined values for `r`, `g` and `b`, this below looks safer:

    ui.r.value = r || 0;
    ui.g.value = g || 0;
    ui.b.value = b || 0;