Add option to stop discovery when predefined lights are found (base branch es2015)

[ristomatti]

I've used a prototype version of this implementation for quite some time to avoid lights dropping out occasionally in my node-red home automation setup.

If for example below settings are passed to client on init, the discovery process will stop after the listed lights have been discovered. This prevents discovery processing setting lights offline if they don't respond for some reason. I've found this does not necessarily prevent packets being processed on the lights.

{
  light: ['192.168.1.10', '192.168.1.11'],
  stopAfterDiscovery: true
}

Client will emit discovery-completed after the lights are found if the configuration option is enabled.

This PR is done against the base branch es2015 like my earlier PR #60 from today.

[codecov-io]

Codecov Report

Merging #61 into es2015 will increase coverage by 0.72%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           es2015      #61      +/-   ##
==========================================
+ Coverage   58.41%   59.13%   +0.72%     
==========================================
  Files          50       50              
  Lines        1688     1713      +25     
  Branches      255      259       +4     
==========================================
+ Hits          986     1013      +27     
+ Misses        702      700       -2

Impacted Files	Coverage Δ
lib/lifx/client.js	90.66% <0%> (+1.57%)	:arrow_up:

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