Add support for text justification property to the :text element.

Proposal:

• new property on :text element: :dir - Contains one of options: :left, :center, :right, :start, :end.
• implementation: preserve existing ctx.textAlign property, set to the value specified by :dir if present (name function will probably help here), restore original property when element is done drawing. This is similar to how :color/:font are applied.
• challenges: None expected.