Official support for latest Crystal versions

• Supporting Crystal 0.35+
• Add Backend::Yaml#exists? to check whether given translation key exists
• Add I18n.exists?
• Add docs to I18n module public methods (most of wording was taken from the ruby I18n repo)

v0.4.0

Update shard.yml

v0.3.1: Update version (#19)

* Update version to 0.3.1

v0.3.0

Use standard for dates.

v0.2.0

Update shard.yml

First release

v0.1.0

Merge branch 'master' of https://github.com/TechMagister/i18n.cr