Adds a shorthand for {{translateAttr}} named {{ta}}

I've been using this shorthand a lot in my recent project and everyone on the team feels that if we use {{t}} for regular translations then {{ta}} for attributes is a no brainer. The pull requests adds support for {{ta}} but also keeps {{translateAttr} and adds tests for {{ta}} itself and a comparison-test so that they don't stray apart in the future.