Add eslint rule to enforce trailing commas in object definitions about blurts-server

[pdehaan]

@nhnt11 Submitted #44, which also added eslint-plugin-node plugin, which should catch deprecated APIs (assuming Node 8+ support, per new package.json engines version).

[pdehaan]

I think that'd be the comma-dangle rule.
Seems infinitely customizable, but this may be a good starting point:

"comma-dangle": ["error", {arrays: "always-multiline", objects: "always-multiline"}],

Or there is the less strict "only-multiline" option as well.

[nhnt11]

Let's do only-multiline. Want to take this? :)