v2.2.1: November 18, 2016

[New] add onMissingKey constructor option - this can call .transformPhrase, or return false, or undefined, or throw - whatever you like (#34, #77)
[Dev Deps] update eslint

[New] Merge choosePluralForm & interpolate into one exported function: transformPhrase (#75)
[New] Allow locales to have regions (#70)
[New] Support Arabic pluralization (#71)
[New] Added Lithuanian locale to russian group of pluralization (#68)
[Deps] update object.assign, warning
[Dev Deps] pin uglify-js because it does not follow semver
[Dev Deps] update eslint-config-airbnb-base, eslint, eslint-plugin-import, mocha, should, uglify-js
[Performance] instead of creating a new regex for each token, use the function form of replace (#19)
[Refactor] use warning package for warnings
[Robustness] Use has package to properly check “has own property”
[Robustness] use string.prototype.trim rather than a homegrown trim polyfill
[Tests] only test on latest node v7, v6, v5, v4, and iojs; improve test matrix
[Tests] add linting

[Tests] up to node v5.1
[Tests] fix npm upgrades on older nodes
[Dev Deps] update uglify-js, docco, should, mocha, and fix test pollution

Added support for nested phrase objects to extend() and in the phrases option in the constructor.

Breaking change: Removed pluralize() method; instead, just use the t() method, passing in a smart_count option.
Breaking change: Removed the ability to use Array, Backbone.Collection, etc. instances for the smart_count option; instead, must pass a Number.
Allow passing Number as second argument to t(), which gets converted to the options object {smart_count: <my number>}.

Added allowMissing option to let the phrase key be the default translation (thanks @ziad-saab).

Breaking change: Moved from Singleton pattern to class-based. Now you create an instance of the Polyglot class rather than using class methods directly on it. The reason is to allow maintaining multiple sets of phrases, which is something we ran into at Airbnb with a highly-concurrent Express app.
Breaking change: Removed the built-in Handlebars helpers, because Handlebars is a singleton, and it’s messy to create a single helper function that can be bound to different Polyglot instances. Instead, it’s super easy to create your own, based on your requirements.