tools: add arrow function rules to eslint

This commit enables the following rules:
* arrow-parens - requires parens around arrow function arguments
* arrow-spacing - ensures a space on each side of the =>
* no-arrow-condition - prevents accidental use of => in cases
where the user really intends to use <=

PR-URL: #4813
Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Saúl Ibarra Corretgé <saghul@gmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>

.eslintrc

@@ -90,6 +90,12 @@ rules:
   # list: http://eslint.org/docs/rules/#ecmascript-6
   ## Suggest using 'const' wherever possible
   prefer-const: 2
+  ## Enforce parens around arrow function arguments
+  arrow-parens: [2, "always"]
+  ## Require a space on each side of arrow operator
+  arrow-spacing: [2, {"before": true, "after": true}]
+  ## Prevent using => in a condition where <= is intended
+  no-arrow-condition: 2
 
   # Strict Mode
   # list: https://github.com/eslint/eslint/tree/master/docs/rules#strict-mode