dump1090-mutability (1.08.2302.14+1mu-3)

• Ask about json interval before json dir and skip the dir question if
  the interval is disabled, it makes more sense that way.
• Be much more cautious about missing config settings so we don't explode
  so badly if something is omitted.
• Use the package version as the version number compiled into the binary.
• Add data/receiver.json (generated once) and support for it in script.js.
• Internal webserver rearrangement to support multiple json files.
• Dongle "device index" actually accepts serial numbers too, document that.
• Add input validation to most of the config questions.