Add JavaScript rule for maximum line length

I'm using the same criteria we use in Ruby: 110 characters, and the
severity set as a warning, since it's a rule we break sometimes.

.eslintrc.yml

@@ -28,6 +28,9 @@ rules:
   keyword-spacing: error
   linebreak-style: error
   lines-between-class-members: error
+  max-len:
+    - warn
+    - code: 110
   no-console: error
   no-multi-spaces: error
   no-multiple-empty-lines: