Added WatchFile gadget to automatically reread files when they change

I created a simple gadget to put ahead of a ReadFileText (or -JSON) gadget to automatically re-read the file when it changes. Seems to work so far, but you may want to hold off merging until I have a little more experience with it.
My booter circuit now has:

````
  gadgets: [
    { name: "jb", type: "JeeBoot" }
    { name: "wf", type: "WatchFile" }
    { name: "rd", type: "ReadFileText" }
...
  wires: [
    { from: "cf.Out", to: "jb.Cfg" }
    { from: "jb.Files", to: "wf.In" }
    { from: "wf.Out", to: "rd.In" }
    { from: "rd.Out", to: "hx.In" }
...